diff --git a/common/collections/stack.go b/common/collections/stack.go
index ff0db2f02..0713d196c 100644
--- a/common/collections/stack.go
+++ b/common/collections/stack.go
@@ -13,9 +13,13 @@
 
 package collections
 
-import "slices"
+import (
+	"iter"
+	"slices"
+	"sync"
 
-import "sync"
+	"github.com/gohugoio/hugo/common/hiter"
+)
 
 // Stack is a simple LIFO stack that is safe for concurrent use.
 type Stack[T any] struct {
@@ -60,6 +64,11 @@ func (s *Stack[T]) Len() int {
 	return len(s.items)
 }
 
+// All returns all items in the stack, from bottom to top.
+func (s *Stack[T]) All() iter.Seq2[int, T] {
+	return hiter.Lock2(slices.All(s.items), s.mu.RLock, s.mu.RUnlock)
+}
+
 func (s *Stack[T]) Drain() []T {
 	s.mu.Lock()
 	defer s.mu.Unlock()
diff --git a/common/hiter/iter.go b/common/hiter/iter.go
index dd221e18c..dd2a41835 100644
--- a/common/hiter/iter.go
+++ b/common/hiter/iter.go
@@ -38,3 +38,29 @@ func Concat2[K, V any](seqs ...iter.Seq2[K, V]) iter.Seq2[K, V] {
 		}
 	}
 }
+
+// Lock returns an iterator that locks before iterating and unlocks after.
+func Lock[V any](seq iter.Seq[V], lock, unlock func()) iter.Seq[V] {
+	return func(yield func(V) bool) {
+		lock()
+		defer unlock()
+		for e := range seq {
+			if !yield(e) {
+				return
+			}
+		}
+	}
+}
+
+// Lock2 returns an iterator that locks before iterating and unlocks after.
+func Lock2[K, V any](seq iter.Seq2[K, V], lock, unlock func()) iter.Seq2[K, V] {
+	return func(yield func(K, V) bool) {
+		lock()
+		defer unlock()
+		for k, v := range seq {
+			if !yield(k, v) {
+				return
+			}
+		}
+	}
+}
diff --git a/hugolib/content_map_page_assembler.go b/hugolib/content_map_page_assembler.go
index a659f1b24..47e03d782 100644
--- a/hugolib/content_map_page_assembler.go
+++ b/hugolib/content_map_page_assembler.go
@@ -14,14 +14,18 @@
 package hugolib
 
 import (
+	"cmp"
 	"context"
 	"fmt"
 	"path"
 	"strings"
 	"time"
 
+	"github.com/gohugoio/hugo/common/maps"
+	"github.com/gohugoio/hugo/common/para"
 	"github.com/gohugoio/hugo/common/paths"
 	"github.com/gohugoio/hugo/common/types"
+	"github.com/gohugoio/hugo/config"
 	"github.com/gohugoio/hugo/hugofs/files"
 	"github.com/gohugoio/hugo/hugolib/doctree"
 	"github.com/gohugoio/hugo/hugolib/sitesmatrix"
@@ -41,14 +45,18 @@ type allPagesAssembler struct {
 	ctx context.Context
 	h   *HugoSites
 	m   *pageMap
+	r   para.Runner
 
-	assembleChanges *WhatChanged
-	seenTerms       map[term]sitesmatrix.Vectors
-	droppedPages    map[*Site][]string // e.g. drafts, expired, future.
+	assembleChanges            *WhatChanged
+	seenTerms                  map[term]sitesmatrix.Vectors
+	droppedPages               map[*Site][]string // e.g. drafts, expired, future.
+	seenRootSections           *maps.Cache[string, bool]
+	seenHome                   bool
+	assembleSectionsInParallel bool
 
 	// Internal state.
-	pw *doctree.NodeShiftTreeWalker[contentNode] // walks pages.
-	rw *doctree.NodeShiftTreeWalker[contentNode] // walks resources.
+	pwRoot *doctree.NodeShiftTreeWalker[contentNode] // walks pages.
+	rwRoot *doctree.NodeShiftTreeWalker[contentNode] // walks resources.
 }
 
 func newAllPagesAssembler(
@@ -66,16 +74,20 @@ func newAllPagesAssembler(
 	pw := rw.Extend()
 	pw.Tree = m.treePages
 
-	return &allPagesAssembler{
-		ctx:             ctx,
-		h:               h,
-		m:               m,
-		assembleChanges: assembleChanges,
-		seenTerms:       map[term]sitesmatrix.Vectors{},
-		droppedPages:    map[*Site][]string{},
+	seenRootSections := maps.NewCache[string, bool]()
+	seenRootSections.Set("", true) // home.
 
-		pw: pw,
-		rw: rw,
+	return &allPagesAssembler{
+		ctx:                        ctx,
+		h:                          h,
+		m:                          m,
+		assembleChanges:            assembleChanges,
+		seenTerms:                  map[term]sitesmatrix.Vectors{},
+		droppedPages:               map[*Site][]string{},
+		seenRootSections:           seenRootSections,
+		assembleSectionsInParallel: true,
+		pwRoot:                     pw,
+		rwRoot:                     rw,
 	}
 }
 
@@ -86,23 +98,7 @@ type sitePagesAssembler struct {
 	ctx             context.Context
 }
 
-// No locking.
 func (a *allPagesAssembler) createAllPages() error {
-	var (
-		sites     = a.h.sitesVersionsRolesMap
-		h         = a.h
-		treePages = a.m.treePages
-
-		getViews = func(vec sitesmatrix.Vector) []viewName {
-			return h.languageSiteForSiteVector(vec).pageMap.cfg.taxonomyConfig.views
-		}
-	)
-
-	resourceOwnerInfo := struct {
-		n contentNode
-		s string
-	}{}
-
 	defer func() {
 		for site, dropped := range a.droppedPages {
 			for _, s := range dropped {
@@ -119,15 +115,15 @@ func (a *allPagesAssembler) createAllPages() error {
 				for t := range a.h.previousSeenTerms {
 					if _, found := a.seenTerms[t]; !found {
 						// This term has been removed.
-						a.pw.Tree.Delete(t.view.pluralTreeKey)
-						a.rw.Tree.DeletePrefix(t.view.pluralTreeKey + "/")
+						a.pwRoot.Tree.Delete(t.view.pluralTreeKey)
+						a.rwRoot.Tree.DeletePrefix(t.view.pluralTreeKey + "/")
 					}
 				}
 				// Find new terms.
 				for t := range a.seenTerms {
 					if _, found := a.h.previousSeenTerms[t]; !found {
 						// This term is new.
-						if n, ok := a.pw.Tree.GetRaw(t.view.pluralTreeKey); ok {
+						if n, ok := a.pwRoot.Tree.GetRaw(t.view.pluralTreeKey); ok {
 							a.assembleChanges.Add(cnh.GetIdentity(n))
 						}
 					}
@@ -136,6 +132,52 @@ func (a *allPagesAssembler) createAllPages() error {
 			a.h.previousSeenTerms = a.seenTerms
 		}()
 	}
+	workers := para.New(config.GetNumWorkerMultiplier())
+	a.r, _ = workers.Start(context.Background())
+	if err := cmp.Or(a.doCreatePages(""), a.r.Wait()); err != nil {
+		return err
+	}
+	if err := a.pwRoot.WalkContext.HandleHooks1AndEventsAndHooks2(); err != nil {
+		return err
+	}
+	return nil
+}
+
+func (a *allPagesAssembler) doCreatePages(prefix string) error {
+	var (
+		sites     = a.h.sitesVersionsRolesMap
+		h         = a.h
+		treePages = a.m.treePages
+
+		getViews = func(vec sitesmatrix.Vector) []viewName {
+			return h.languageSiteForSiteVector(vec).pageMap.cfg.taxonomyConfig.views
+		}
+
+		pw *doctree.NodeShiftTreeWalker[contentNode] // walks pages.
+		rw *doctree.NodeShiftTreeWalker[contentNode] // walks resources.
+
+		isRootWalk = prefix == ""
+	)
+
+	if isRootWalk {
+		pw = a.pwRoot
+		rw = a.rwRoot
+	} else {
+		// Sub-walkers for a specific prefix.
+		pw = a.pwRoot.Extend()
+		pw.Prefix = prefix + "/"
+
+		rw = a.rwRoot.Extend() // rw will get its prefix(es) set later.
+
+		pw.TransformDelayInsert = true
+		rw.TransformDelayInsert = true
+
+	}
+
+	resourceOwnerInfo := struct {
+		n contentNode
+		s string
+	}{}
 
 	newHomePageMetaSource := func() *pageMetaSource {
 		pi := a.h.Conf.PathParser().Parse(files.ComponentFolderContent, "/_index.md")
@@ -149,14 +191,16 @@ func (a *allPagesAssembler) createAllPages() error {
 		}
 	}
 
-	if err := a.createMissingPages(); err != nil {
-		return err
-	}
+	if isRootWalk {
+		if err := a.createMissingPages(); err != nil {
+			return err
+		}
 
-	if treePages.Len() == 0 {
-		// No pages, insert a home page to get something to walk on.
-		p := newHomePageMetaSource()
-		treePages.InsertRaw(p.pathInfo.Base(), p)
+		if treePages.Len() == 0 {
+			// No pages, insert a home page to get something to walk on.
+			p := newHomePageMetaSource()
+			treePages.InsertRaw(p.pathInfo.Base(), p)
+		}
 	}
 
 	getCascades := func(wctx *doctree.WalkContext[contentNode], s string) *page.PageMatcherParamsConfigs {
@@ -201,7 +245,7 @@ func (a *allPagesAssembler) createAllPages() error {
 
 			if s == "" || cascades.Len() > cascadesLen {
 				// New cascade values added, pass them downwards.
-				a.rw.WalkContext.Data().Insert(paths.AddTrailingSlash(s), cascades)
+				rw.WalkContext.Data().Insert(paths.AddTrailingSlash(s), cascades)
 			}
 		}()
 
@@ -351,13 +395,6 @@ func (a *allPagesAssembler) createAllPages() error {
 		}
 	}
 
-	var (
-		seenRootSections = map[string]bool{
-			"": true, // The home section.
-		}
-		seenHome bool
-	)
-
 	var unpackPageMetaSources func(n contentNode) contentNode
 	unpackPageMetaSources = func(n contentNode) contentNode {
 		switch nn := n.(type) {
@@ -430,18 +467,17 @@ func (a *allPagesAssembler) createAllPages() error {
 		level := strings.Count(s, "/")
 
 		if s == "" {
-			seenHome = true
+			a.seenHome = true
 		}
 
-		if !isResource && s != "" && !seenHome {
-
+		if !isResource && s != "" && !a.seenHome {
 			var homePages contentNode
 			homePages, ns, err = transformPages("", newHomePageMetaSource(), cascades)
 			if err != nil {
 				return
 			}
 			treePages.InsertRaw("", homePages)
-			seenHome = true
+			a.seenHome = true
 		}
 
 		n2, ns, err = transformPages(s, n, cascades)
@@ -460,16 +496,15 @@ func (a *allPagesAssembler) createAllPages() error {
 		}
 
 		isTaxonomy := !a.h.getFirstTaxonomyConfig(s).IsZero()
-		isRootSetion := !isTaxonomy && level == 1 && cnh.isBranchNode(n)
+		isRootSection := !isTaxonomy && level == 1 && cnh.isBranchNode(n)
 
-		if isRootSetion {
+		if isRootSection {
 			// This is a root section.
-			seenRootSections[cnh.PathInfo(n).Section()] = true
+			a.seenRootSections.SetIfAbsent(cnh.PathInfo(n).Section(), true)
 		} else if !isTaxonomy {
 			p := cnh.PathInfo(n)
 			rootSection := p.Section()
-			if !seenRootSections[rootSection] {
-				seenRootSections[rootSection] = true
+			_, err := a.seenRootSections.GetOrCreate(rootSection, func() (bool, error) {
 				// Try to preserve the original casing if possible.
 				sectionUnnormalized := p.Unnormalized().Section()
 				rootSectionPath := a.h.Conf.PathParser().Parse(files.ComponentFolderContent, "/"+sectionUnnormalized+"/_index.md")
@@ -482,15 +517,20 @@ func (a *allPagesAssembler) createAllPages() error {
 					},
 				}, cascades)
 				if err != nil {
-					return
+					return true, err
 				}
 				treePages.InsertRaw(rootSectionPath.Base(), rootSectionPages)
+				return true, nil
+			})
+			if err != nil {
+				return nil, 0, err
 			}
+
 		}
 
 		const eventNameSitesMatrix = "sitesmatrix"
 
-		if s == "" || isRootSetion {
+		if s == "" || isRootSection {
 
 			// Every page needs a home and a root section (.FirstSection).
 			// We don't know yet what language, version, role combination that will
@@ -509,7 +549,7 @@ func (a *allPagesAssembler) createAllPages() error {
 					return true
 				})
 			} else {
-				a.pw.WalkContext.AddEventListener(eventNameSitesMatrix, s,
+				pw.WalkContext.AddEventListener(eventNameSitesMatrix, s,
 					func(e *doctree.Event[contentNode]) {
 						n := e.Source
 						e.StopPropagation()
@@ -525,7 +565,7 @@ func (a *allPagesAssembler) createAllPages() error {
 			}
 
 			// We need to wait until after the walk to have a complete set.
-			a.pw.WalkContext.AddPostHook(
+			pw.WalkContext.HooksPost2().Push(
 				func() error {
 					if i := len(missingVectorsForHomeOrRootSection); i > 0 {
 						// Pick one, the rest will be created later.
@@ -556,7 +596,7 @@ func (a *allPagesAssembler) createAllPages() error {
 						}
 
 						if replaced {
-							a.pw.Tree.InsertRaw(s, nm)
+							pw.Tree.InsertRaw(s, nm)
 						}
 					}
 					return nil
@@ -565,14 +605,14 @@ func (a *allPagesAssembler) createAllPages() error {
 		}
 
 		if s != "" {
-			a.rw.WalkContext.SendEvent(&doctree.Event[contentNode]{Source: n2, Path: s, Name: eventNameSitesMatrix})
+			rw.WalkContext.SendEvent(&doctree.Event[contentNode]{Source: n2, Path: s, Name: eventNameSitesMatrix})
 		}
 
 		return
 	}
 
 	transformPagesAndCreateMissingStructuralNodes := func(s string, n contentNode, isResource bool) (n2 contentNode, ns doctree.NodeTransformState, err error) {
-		cascades := getCascades(a.pw.WalkContext, s)
+		cascades := getCascades(pw.WalkContext, s)
 		n2, ns, err = transformPagesAndCreateMissingHome(s, n, isResource, cascades)
 		if err != nil || ns >= doctree.NodeTransformStateSkip {
 			return
@@ -643,7 +683,7 @@ func (a *allPagesAssembler) createAllPages() error {
 			}
 
 			// Stop walking downwards, someone else owns this resource.
-			a.rw.SkipPrefix(ownerKey + "/")
+			rw.SkipPrefix(ownerKey + "/")
 			return doctree.NodeTransformStateSkip
 		}
 		return
@@ -665,7 +705,7 @@ func (a *allPagesAssembler) createAllPages() error {
 		return true
 	}
 
-	a.rw.Transform = func(s string, n contentNode) (n2 contentNode, ns doctree.NodeTransformState, err error) {
+	rw.Transform = func(s string, n contentNode) (n2 contentNode, ns doctree.NodeTransformState, err error) {
 		if ns = shouldSkipOrTerminate(s); ns >= doctree.NodeTransformStateSkip {
 			return
 		}
@@ -710,7 +750,7 @@ func (a *allPagesAssembler) createAllPages() error {
 	}
 
 	// Create  missing term pages.
-	a.pw.WalkContext.AddPostHook(
+	pw.WalkContext.HooksPost2().Push(
 		func() error {
 			for k, v := range a.seenTerms {
 				viewTermKey := "/" + k.view.plural + "/" + k.term
@@ -718,7 +758,7 @@ func (a *allPagesAssembler) createAllPages() error {
 				pi := a.h.Conf.PathParser().Parse(files.ComponentFolderContent, viewTermKey+"/_index.md")
 				termKey := pi.Base()
 
-				n, found := a.pw.Tree.GetRaw(termKey)
+				n, found := pw.Tree.GetRaw(termKey)
 
 				if found {
 					// Merge.
@@ -742,14 +782,14 @@ func (a *allPagesAssembler) createAllPages() error {
 					if found {
 						n2 = contentNodes{n, p}
 					}
-					n2, ns, err := transformPages(termKey, n2, getCascades(a.pw.WalkContext, termKey))
+					n2, ns, err := transformPages(termKey, n2, getCascades(pw.WalkContext, termKey))
 					if err != nil {
 						return fmt.Errorf("failed to create term page %q: %w", termKey, err)
 					}
 
 					switch ns {
 					case doctree.NodeTransformStateReplaced:
-						a.pw.Tree.InsertRaw(termKey, n2)
+						pw.Tree.InsertRaw(termKey, n2)
 					}
 
 				}
@@ -758,7 +798,7 @@ func (a *allPagesAssembler) createAllPages() error {
 		},
 	)
 
-	a.pw.Transform = func(s string, n contentNode) (n2 contentNode, ns doctree.NodeTransformState, err error) {
+	pw.Transform = func(s string, n contentNode) (n2 contentNode, ns doctree.NodeTransformState, err error) {
 		n2, ns, err = transformPagesAndCreateMissingStructuralNodes(s, n, false)
 
 		if err != nil || ns >= doctree.NodeTransformStateSkip {
@@ -776,21 +816,25 @@ func (a *allPagesAssembler) createAllPages() error {
 		// Walk nested resources.
 		resourceOwnerInfo.s = s
 		resourceOwnerInfo.n = n2
-		a.rw.Prefix = s + "/"
-		if err := a.rw.Walk(a.ctx); err != nil {
+		rw = rw.WithPrefix(s + "/")
+		if err := rw.Walk(a.ctx); err != nil {
 			return nil, 0, err
 		}
 
+		if a.assembleSectionsInParallel && prefix == "" && s != "" && cnh.isBranchNode(n) && a.h.getFirstTaxonomyConfig(s).IsZero() {
+			// Handle this branch's descendants in its own goroutine.
+			pw.SkipPrefix(s + "/")
+			a.r.Run(func() error {
+				return a.doCreatePages(s)
+			})
+		}
+
 		return
 	}
 
-	a.pw.Handle = nil
+	pw.Handle = nil
 
-	if err := a.pw.Walk(a.ctx); err != nil {
-		return err
-	}
-
-	if err := a.pw.WalkContext.HandleEventsAndHooks(); err != nil {
+	if err := pw.Walk(a.ctx); err != nil {
 		return err
 	}
 
@@ -829,7 +873,7 @@ func (sa *sitePagesAssembler) applyAggregates() error {
 				oldDates := pageBundle.m.pageConfig.Dates
 
 				// We need to wait until after the walk to determine if any of the dates have changed.
-				pw.WalkContext.AddPostHook(
+				pw.WalkContext.HooksPost2().Push(
 					func() error {
 						if oldDates != pageBundle.m.pageConfig.Dates {
 							sa.assembleChanges.Add(pageBundle)
@@ -910,7 +954,7 @@ func (sa *sitePagesAssembler) applyAggregates() error {
 		return err
 	}
 
-	if err := pw.WalkContext.HandleEventsAndHooks(); err != nil {
+	if err := pw.WalkContext.HandleHooks1AndEventsAndHooks2(); err != nil {
 		return err
 	}
 
@@ -997,7 +1041,7 @@ func (sa *sitePagesAssembler) applyAggregatesToTaxonomiesAndTerms() error {
 		}
 	}
 
-	if err := walkContext.HandleEventsAndHooks(); err != nil {
+	if err := walkContext.HandleHooks1AndEventsAndHooks2(); err != nil {
 		return err
 	}
 
diff --git a/hugolib/content_map_page_contentnodeshifter.go b/hugolib/content_map_page_contentnodeshifter.go
index b9d183ddd..c026997fc 100644
--- a/hugolib/content_map_page_contentnodeshifter.go
+++ b/hugolib/content_map_page_contentnodeshifter.go
@@ -146,7 +146,7 @@ func (s *contentNodeShifter) Shift(n contentNode, siteVector sitesmatrix.Vector,
 			return vv, true
 		}
 	default:
-		panic(fmt.Sprintf("Shift: unknown type %T", n))
+		panic(fmt.Sprintf("Shift: unknown type %T for %q", n, n.Path()))
 	}
 
 	if !fallback {
diff --git a/hugolib/doctree/nodeshiftree_test.go b/hugolib/doctree/nodeshiftree_test.go
index 74ce26043..b75586ce1 100644
--- a/hugolib/doctree/nodeshiftree_test.go
+++ b/hugolib/doctree/nodeshiftree_test.go
@@ -147,7 +147,7 @@ func TestTreeEvents(t *testing.T) {
 	}
 
 	c.Assert(w.Walk(context.Background()), qt.IsNil)
-	c.Assert(w.WalkContext.HandleEventsAndHooks(), qt.IsNil)
+	c.Assert(w.WalkContext.HandleHooks1AndEventsAndHooks2(), qt.IsNil)
 
 	c.Assert(tree.Get("/a").Weight, eq, 9)
 	c.Assert(tree.Get("/a/s1").Weight, eq, 9)
diff --git a/hugolib/doctree/nodeshifttree.go b/hugolib/doctree/nodeshifttree.go
index aa1b6a723..8f64be396 100644
--- a/hugolib/doctree/nodeshifttree.go
+++ b/hugolib/doctree/nodeshifttree.go
@@ -381,6 +381,9 @@ type NodeShiftTreeWalker[T any] struct {
 	// the third bool indicates if the walk should terminate.
 	Transform func(s string, v1 T) (v2 T, state NodeTransformState, err error)
 
+	// When set, will add inserts to WalkContext.HooksPost1 to be performed after the walk.
+	TransformDelayInsert bool
+
 	// Handle will be called for each node in the main tree.
 	// If the callback returns true, the walk will stop.
 	// The callback can optionally return a callback for the nested tree.
@@ -405,8 +408,8 @@ type NodeShiftTreeWalker[T any] struct {
 	// When set, no dimension shifting will be performed.
 	NoShift bool
 
-	// WHen set, will try to fall back to alternative match,
-	// typically a shared resoures common for all languages.
+	// When set, will try to fall back to alternative match,
+	// typically a shared resource common for all languages.
 	Fallback bool
 
 	// Used in development only.
@@ -423,14 +426,21 @@ type NodeShiftTreeWalker[T any] struct {
 	skipPrefixes []string
 }
 
-// Extend returns a new NodeShiftTreeWalker with the same configuration as the
+// Extend returns a new NodeShiftTreeWalker with the same configuration
 // and the same WalkContext as the original.
 // Any local state is reset.
 func (r NodeShiftTreeWalker[T]) Extend() *NodeShiftTreeWalker[T] {
-	r.resetLocalState()
+	r.skipPrefixes = nil
 	return &r
 }
 
+// WithPrefix returns a new NodeShiftTreeWalker with the given prefix.
+func (r *NodeShiftTreeWalker[T]) WithPrefix(prefix string) *NodeShiftTreeWalker[T] {
+	r2 := r.Extend()
+	r2.Prefix = prefix
+	return r2
+}
+
 // SkipPrefix adds a prefix to be skipped in the walk.
 func (r *NodeShiftTreeWalker[T]) SkipPrefix(prefix ...string) {
 	r.skipPrefixes = append(r.skipPrefixes, prefix...)
@@ -474,7 +484,18 @@ func (r *NodeShiftTreeWalker[T]) Walk(ctx context.Context) error {
 
 				switch ns {
 				case NodeTransformStateReplaced:
-					r.Tree.tree.Insert(s, t)
+					// Delay insert until after the walk to
+					// avoid locking issues.
+					if r.TransformDelayInsert {
+						r.WalkContext.HooksPost1().Push(
+							func() error {
+								r.Tree.InsertRaw(s, t)
+								return nil
+							},
+						)
+					} else {
+						r.Tree.InsertRaw(s, t)
+					}
 				case NodeTransformStateDeleted:
 					// Delay delete until after the walk.
 					deletes = append(deletes, s)
diff --git a/hugolib/doctree/support.go b/hugolib/doctree/support.go
index 9cdc36fda..3692b1167 100644
--- a/hugolib/doctree/support.go
+++ b/hugolib/doctree/support.go
@@ -19,6 +19,7 @@ import (
 	"strings"
 	"sync"
 
+	"github.com/gohugoio/hugo/common/collections"
 	"github.com/gohugoio/hugo/common/maps"
 	"github.com/gohugoio/hugo/hugolib/sitesmatrix"
 )
@@ -34,6 +35,9 @@ const (
 // AddEventListener adds an event listener to the tree.
 // Note that the handler func may not add listeners.
 func (ctx *WalkContext[T]) AddEventListener(event, path string, handler func(*Event[T])) {
+	ctx.eventHandlersMu.Lock()
+	defer ctx.eventHandlersMu.Unlock()
+
 	if ctx.eventHandlers == nil {
 		ctx.eventHandlers = make(eventHandlers[T])
 	}
@@ -56,12 +60,6 @@ func (ctx *WalkContext[T]) AddEventListener(event, path string, handler func(*Ev
 	)
 }
 
-// AddPostHook adds a post hook to the tree.
-// This will be run after the tree has been walked.
-func (ctx *WalkContext[T]) AddPostHook(handler func() error) {
-	ctx.HooksPost = append(ctx.HooksPost, handler)
-}
-
 func (ctx *WalkContext[T]) Data() *SimpleThreadSafeTree[any] {
 	ctx.dataInit.Do(func() {
 		ctx.data = NewSimpleThreadSafeTree[any]()
@@ -94,6 +92,8 @@ func (ctx *WalkContext[T]) DataRawForEeach() iter.Seq2[sitesmatrix.Vector, *Simp
 
 // SendEvent sends an event up the tree.
 func (ctx *WalkContext[T]) SendEvent(event *Event[T]) {
+	ctx.eventMu.Lock()
+	defer ctx.eventMu.Unlock()
 	ctx.events = append(ctx.events, event)
 }
 
@@ -213,10 +213,16 @@ type WalkContext[T any] struct {
 	dataRaw     *maps.Cache[sitesmatrix.Vector, *SimpleThreadSafeTree[any]]
 	dataRawInit sync.Once
 
-	eventHandlers eventHandlers[T]
-	events        []*Event[T]
+	eventHandlersMu sync.Mutex
+	eventHandlers   eventHandlers[T]
+	eventMu         sync.Mutex
+	events          []*Event[T]
 
-	HooksPost []func() error
+	hooksPost1Init sync.Once
+	hooksPost1     *collections.Stack[func() error]
+
+	hooksPost2Init sync.Once
+	hooksPost2     *collections.Stack[func() error]
 }
 
 type eventHandlers[T any] map[string][]func(*Event[T])
@@ -233,6 +239,9 @@ func cleanKey(key string) string {
 }
 
 func (ctx *WalkContext[T]) HandleEvents() error {
+	ctx.eventHandlersMu.Lock()
+	defer ctx.eventHandlersMu.Unlock()
+
 	for len(ctx.events) > 0 {
 		event := ctx.events[0]
 		ctx.events = ctx.events[1:]
@@ -250,12 +259,32 @@ func (ctx *WalkContext[T]) HandleEvents() error {
 	return nil
 }
 
-func (ctx *WalkContext[T]) HandleEventsAndHooks() error {
+func (ctx *WalkContext[T]) HooksPost1() *collections.Stack[func() error] {
+	ctx.hooksPost1Init.Do(func() {
+		ctx.hooksPost1 = collections.NewStack[func() error]()
+	})
+	return ctx.hooksPost1
+}
+
+func (ctx *WalkContext[T]) HooksPost2() *collections.Stack[func() error] {
+	ctx.hooksPost2Init.Do(func() {
+		ctx.hooksPost2 = collections.NewStack[func() error]()
+	})
+	return ctx.hooksPost2
+}
+
+func (ctx *WalkContext[T]) HandleHooks1AndEventsAndHooks2() error {
+	for _, hook := range ctx.HooksPost1().All() {
+		if err := hook(); err != nil {
+			return err
+		}
+	}
+
 	if err := ctx.HandleEvents(); err != nil {
 		return err
 	}
 
-	for _, hook := range ctx.HooksPost {
+	for _, hook := range ctx.HooksPost2().All() {
 		if err := hook(); err != nil {
 			return err
 		}
diff --git a/hugolib/hugo_sites_build.go b/hugolib/hugo_sites_build.go
index b0157bb59..87321d471 100644
--- a/hugolib/hugo_sites_build.go
+++ b/hugolib/hugo_sites_build.go
@@ -336,7 +336,7 @@ func (h *HugoSites) assemble(ctx context.Context, l logg.LevelLogger, bcfg *Buil
 	if h.Conf.Watching() {
 		defer func() {
 			// Store previous walk context to detect cascade changes on next rebuild.
-			h.previousPageTreesWalkContext = apa.rw.WalkContext
+			h.previousPageTreesWalkContext = apa.rwRoot.WalkContext
 		}()
 	}
 
diff --git a/hugolib/hugo_sites_build_test.go b/hugolib/hugo_sites_build_test.go
index 29cdaa482..4ef48e6f4 100644
--- a/hugolib/hugo_sites_build_test.go
+++ b/hugolib/hugo_sites_build_test.go
@@ -204,6 +204,7 @@ func BenchmarkAssembleDeepSiteWithManySections(b *testing.B) {
 		})
 	}
 
+	runOne(1, 1, 20)
 	runOne(1, 6, 100)
 	runOne(2, 2, 100)
 	runOne(2, 6, 100)
diff --git a/hugolib/sitesmatrix/sitematrix_integration_test.go b/hugolib/sitesmatrix/sitematrix_integration_test.go
index 048fcc791..1b1174614 100644
--- a/hugolib/sitesmatrix/sitematrix_integration_test.go
+++ b/hugolib/sitesmatrix/sitematrix_integration_test.go
@@ -1231,3 +1231,27 @@ func BenchmarkSitesMatrixContent(b *testing.B) {
 		}
 	}
 }
+
+// Just a test to test the development of concurrency in page assembly.
+func TestCreateAllPagesPartitionSections(t *testing.T) {
+	files := `
+-- hugo.toml --
+baseURL = "https://example.org/"
+disableKinds = ["rss", "sitemap", "taxonomy", "term"]
+-- content/_index.md --
+-- content/s1/_index.md --
+-- content/s1/p1.md --
+-- content/s1/p2.md --
+-- content/s2/_index.md --
+-- content/s2/p1.md --
+-- content/s2_1/_index.md --
+-- content/s2_1/p1.md --
+-- layouts/all.html --
+{{ .Kind }}|{{ .RelPermalink }}|
+`
+
+	for range 3 {
+		b := hugolib.Test(t, files)
+		b.AssertFileContent("public/s1/index.html", "section|/s1/|")
+	}
+}