perf: avoid re-traversing the effect tree after `$:` assignments

.changeset/ninety-kings-attend.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+perf: avoid re-traversing the effect tree after `$:` assignments

packages/svelte/src/internal/client/reactivity/batch.js

@@ -23,6 +23,7 @@ import { async_mode_flag } from '../../flags/index.js';
 import { deferred, define_property, includes } from '../../shared/utils.js';
 import {
 	active_effect,
+	active_reaction,
 	get,
 	increment_write_version,
 	is_dirty,
@@ -860,7 +861,7 @@ export function schedule_effect(signal) {
 			// in sync mode, render effects run during traversal. in an extreme edge case
 			// they can be made dirty after they have already been visited, in which
 			// case we shouldn't bail out
-			if (async_mode_flag || (signal.f & RENDER_EFFECT) === 0) {
+			if (async_mode_flag || active_reaction === null || (active_reaction.f & DERIVED) === 0) {
 				return;
 			}
 		}

packages/svelte/src/internal/client/reactivity/effects.js

@@ -10,7 +10,8 @@ import {
 	set_active_reaction,
 	set_is_destroying_effect,
 	untrack,
-	untracking
+	untracking,
+	set_active_effect
 } from '../runtime.js';
 import {
 	DIRTY,
@@ -316,7 +317,19 @@ export function legacy_pre_effect(deps, fn) {
 		if (token.ran) return;
 
 		token.ran = true;
-		untrack(fn);
+
+		var effect = /** @type {Effect} */ (active_effect);
+
+		// here, we lie: by setting `active_effect` to be the parent branch, any writes
+		// that happen inside `fn` will _not_ cause an unnecessary reschedule, because
+		// the affected effects will be children of `active_effect`. this is safe
+		// because these effects are known to run in the correct order
+		try {
+			set_active_effect(effect.parent);
+			untrack(fn);
+		} finally {
+			set_active_effect(effect);
+		}
 	});
 }