diff --git a/.changeset/current-sources-set.md b/.changeset/current-sources-set.md
new file mode 100644
index 0000000000..e1a4dc43b2
--- /dev/null
+++ b/.changeset/current-sources-set.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+perf: store `current_sources` as a `Set` for O(1) membership checks
diff --git a/packages/svelte/src/internal/client/reactivity/sources.js b/packages/svelte/src/internal/client/reactivity/sources.js
index f374f6a26b..218941ab67 100644
--- a/packages/svelte/src/internal/client/reactivity/sources.js
+++ b/packages/svelte/src/internal/client/reactivity/sources.js
@@ -32,7 +32,6 @@ import {
 } from '#client/constants';
 import * as e from '../errors.js';
 import { legacy_mode_flag, tracing_mode_flag } from '../../flags/index.js';
-import { includes } from '../../shared/utils.js';
 import { tag_proxy } from '../dev/tracing.js';
 import { get_error } from '../../shared/dev.js';
 import { component_context, is_runes } from '../context.js';
@@ -158,7 +157,7 @@ export function set(source, value, should_proxy = false) {
 		(!untracking || (active_reaction.f & EAGER_EFFECT) !== 0) &&
 		is_runes() &&
 		(active_reaction.f & (DERIVED | BLOCK_EFFECT | ASYNC | EAGER_EFFECT)) !== 0 &&
-		(current_sources === null || !includes.call(current_sources, source))
+		(current_sources === null || !current_sources.has(source))
 	) {
 		e.state_unsafe_mutation();
 	}
diff --git a/packages/svelte/src/internal/client/runtime.js b/packages/svelte/src/internal/client/runtime.js
index 87abbd71b6..8d70eee43c 100644
--- a/packages/svelte/src/internal/client/runtime.js
+++ b/packages/svelte/src/internal/client/runtime.js
@@ -90,18 +90,14 @@ export function set_active_effect(effect) {
 /**
  * When sources are created within a reaction, reading and writing
  * them within that reaction should not cause a re-run
- * @type {null | Source[]}
+ * @type {null | Set<Source>}
  */
 export let current_sources = null;
 
 /** @param {Value} value */
 export function push_reaction_value(value) {
 	if (active_reaction !== null && (!async_mode_flag || (active_reaction.f & DERIVED) !== 0)) {
-		if (current_sources === null) {
-			current_sources = [value];
-		} else {
-			current_sources.push(value);
-		}
+		(current_sources ??= new Set()).add(value);
 	}
 }
 
@@ -202,7 +198,7 @@ function schedule_possible_effect_self_invalidation(signal, effect, root = true)
 	var reactions = signal.reactions;
 	if (reactions === null) return;
 
-	if (!async_mode_flag && current_sources !== null && includes.call(current_sources, signal)) {
+	if (!async_mode_flag && current_sources !== null && current_sources.has(signal)) {
 		return;
 	}
 
@@ -540,7 +536,7 @@ export function get(signal) {
 		// we don't add the dependency, because that would create a memory leak
 		var destroyed = active_effect !== null && (active_effect.f & DESTROYED) !== 0;
 
-		if (!destroyed && (current_sources === null || !includes.call(current_sources, signal))) {
+		if (!destroyed && (current_sources === null || !current_sources.has(signal))) {
 			var deps = active_reaction.deps;
 
 			if ((active_reaction.f & REACTION_IS_UPDATING) !== 0) {