diff --git a/changes/unreleased/memoize-short-named.performance.md b/changes/unreleased/memoize-short-named.performance.md
new file mode 100644
index 000000000..5ce7370a7
--- /dev/null
+++ b/changes/unreleased/memoize-short-named.performance.md
@@ -0,0 +1 @@
+- Validating a model rich in membership imports no longer re-scans the names registered under a segment on every lookup: `ShortNamed` is memoized per index generation, undoing a ~25% whole-model validate slowdown introduced with the import-prune fix.
diff --git a/internal/semantic/symbols/index.go b/internal/semantic/symbols/index.go
index 173430424..bb15efa71 100644
--- a/internal/semantic/symbols/index.go
+++ b/internal/semantic/symbols/index.go
@@ -48,6 +48,9 @@ type Index struct {
 	directChildrenGeneration uint64
 	directChildrenCache      map[directChildrenKey][]*Symbol
 	directChildrenByName     map[directChildrenKey]map[string][]*Symbol
+	// shortNamedCache memoizes ShortNamed per segment, reset with the
+	// direct-children caches.
+	shortNamedCache map[string]bool
 
 	docRoots      *layer[string, *Scope]      // document name -> root scope
 	docOfRoot     *layer[*Scope, string]      // root scope -> document name
@@ -193,6 +196,7 @@ func NewIndex() *Index {
 		generation:           gen,
 		directChildrenCache:  make(map[directChildrenKey][]*Symbol),
 		directChildrenByName: make(map[directChildrenKey]map[string][]*Symbol),
+		shortNamedCache:      make(map[string]bool),
 		docRoots:             newLayer[string, *Scope](gen),
 		docOfRoot:            newLayer[*Scope, string](gen),
 		docKinds:             newLayer[string, source.Kind](gen),
@@ -311,6 +315,7 @@ func NewOverlay(base *Index) *Index {
 		generation:           gen,
 		directChildrenCache:  make(map[directChildrenKey][]*Symbol),
 		directChildrenByName: make(map[directChildrenKey]map[string][]*Symbol),
+		shortNamedCache:      make(map[string]bool),
 		docRoots:             overLayer(base.docRoots, gen),
 		docOfRoot:            overLayer(base.docOfRoot, gen),
 		docKinds:             overLayer(base.docKinds, gen),
@@ -1590,6 +1595,24 @@ func (idx *Index) ShortNamed(name string) bool {
 		return false
 	}
 	idx.readSegment(name)
+	generation := idx.generation.get()
+	idx.directChildrenMu.Lock()
+	idx.resetDirectChildrenCachesLocked(generation)
+	if v, ok := idx.shortNamedCache[name]; ok {
+		idx.directChildrenMu.Unlock()
+		return v
+	}
+	idx.directChildrenMu.Unlock()
+	v := idx.shortNamedScan(name)
+	idx.directChildrenMu.Lock()
+	if idx.generation.get() == generation {
+		idx.shortNamedCache[name] = v
+	}
+	idx.directChildrenMu.Unlock()
+	return v
+}
+
+func (idx *Index) shortNamedScan(name string) bool {
 	shortRegistered := func(fqn string) bool {
 		for _, sym := range idx.fqn.at(fqn) {
 			if LastSegment(sym.Name) != name {
@@ -1712,6 +1735,7 @@ func (idx *Index) resetDirectChildrenCachesLocked(generation uint64) {
 	if idx.directChildrenGeneration != generation {
 		idx.directChildrenCache = make(map[directChildrenKey][]*Symbol)
 		idx.directChildrenByName = make(map[directChildrenKey]map[string][]*Symbol)
+		idx.shortNamedCache = make(map[string]bool)
 		idx.directChildrenGeneration = generation
 	}
 }
diff --git a/internal/semantic/symbols/short_named_test.go b/internal/semantic/symbols/short_named_test.go
new file mode 100644
index 000000000..e1df01195
--- /dev/null
+++ b/internal/semantic/symbols/short_named_test.go
@@ -0,0 +1,31 @@
+package symbols
+
+import "testing"
+
+func TestShortNamedFollowsIndexGeneration(t *testing.T) {
+	idx := NewIndex()
+	if idx.ShortNamed("s") {
+		t.Fatal("ShortNamed(s) = true on an empty index")
+	}
+	if idx.ShortNamed("s") {
+		t.Fatal("ShortNamed(s) changed on a repeated call of an unchanged index")
+	}
+
+	addDoc(t, idx, "a.sysml", "package P { part def <s> Y; }")
+	if !idx.ShortNamed("s") {
+		t.Fatal("ShortNamed(s) = false after adding a short-named member (stale cache)")
+	}
+	if !idx.ShortNamed("s") {
+		t.Fatal("ShortNamed(s) changed on a repeated call of an unchanged index")
+	}
+
+	addDoc(t, idx, "a.sysml", "package P { part def Z; }")
+	if idx.ShortNamed("s") {
+		t.Fatal("ShortNamed(s) = true after the document dropped the short name (stale cache)")
+	}
+
+	idx.RemoveDocument("a.sysml")
+	if idx.ShortNamed("s") {
+		t.Fatal("ShortNamed(s) = true after removing the document (stale cache)")
+	}
+}
diff --git a/internal/semantic/symbols/snapshot_test.go b/internal/semantic/symbols/snapshot_test.go
index 4f3812613..d489e67d4 100644
--- a/internal/semantic/symbols/snapshot_test.go
+++ b/internal/semantic/symbols/snapshot_test.go
@@ -14,7 +14,7 @@ import (
 // name leaves behind once its parts are appended elsewhere.
 var unobservable = graphcmp.SkipFields(
 	"Index.directChildrenGeneration", "libraryIdentityMemo.gen",
-	"Index.directChildrenCache", "Index.directChildrenByName",
+	"Index.directChildrenCache", "Index.directChildrenByName", "Index.shortNamedCache",
 	"QualifiedName.part0",
 )
 
diff --git a/internal/workspace/libs/snapshot_test.go b/internal/workspace/libs/snapshot_test.go
index b28172cfe..861dbba9e 100644
--- a/internal/workspace/libs/snapshot_test.go
+++ b/internal/workspace/libs/snapshot_test.go
@@ -54,7 +54,7 @@ func TestSnapshotIndexMatchesFreshLoad(t *testing.T) {
 	// the inline storage a multi-part name leaves behind (see symbols' tests).
 	if err := graphcmp.Equal(fresh, decoded, graphcmp.SkipFields(
 		"Index.directChildrenGeneration", "libraryIdentityMemo.gen", "Index.directChildrenCache",
-		"Index.directChildrenByName", "QualifiedName.part0",
+		"Index.directChildrenByName", "Index.shortNamedCache", "QualifiedName.part0",
 	)); err != nil {
 		t.Errorf("decoded index differs from a fresh load: %v", err)
 	}