diff --git a/.changeset/flat-moose-arrive.md b/.changeset/flat-moose-arrive.md
new file mode 100644
index 0000000000..0a8c229a66
--- /dev/null
+++ b/.changeset/flat-moose-arrive.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: avoid extra work in mark_reactions
diff --git a/packages/svelte/src/internal/client/reactivity/sources.js b/packages/svelte/src/internal/client/reactivity/sources.js
index 86427a7509..772d62d026 100644
--- a/packages/svelte/src/internal/client/reactivity/sources.js
+++ b/packages/svelte/src/internal/client/reactivity/sources.js
@@ -145,8 +145,8 @@ function mark_reactions(signal, status) {
 		var reaction = reactions[i];
 		var flags = reaction.f;
 
-		// Skip any effects that are already dirty
-		if ((flags & DIRTY) !== 0) continue;
+		// If a reaction is already dirty, skip it (but always mark unowned deriveds)
+		if ((flags & (CLEAN | UNOWNED)) === 0) continue;
 
 		// In legacy mode, skip the current effect to prevent infinite loops
 		if (!runes && reaction === current_effect) continue;