We need to use `deep_read` to traverse all properties of an object inside legacy mode and inside actions, because the input we get could be a state proxy. This proxy will have fine-grained reactivity and therefore only update the signal at the location that got an update. In order to have interop with the coarse-grained "notify me when _anything_ in that object changes" behavior, we need to setup up signals for all of the object's properties before they're potentially mutated.

This has a lot of overhead for large lists, and we can at least diminish in the "no state proxy" case by applying a sensible heuristic:
- If the value passed is a state proxy, read it
- If not, and if the value is an array, then bail because an array of state proxies is highly unlikely
- Traverse the first level of properties of the object and look if these are state, if not bail. State proxies nested further down are highly unlikely, too

part of #10637

.changeset/chatty-sloths-allow.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+perf: bail early when traversing non-state

packages/svelte/src/compiler/phases/3-transform/client/visitors/javascript-legacy.js

@@ -170,7 +170,7 @@ export const javascript_visitors_legacy = {
 			// If the binding is a prop, we need to deep read it because it could be fine-grained $state
 			// from a runes-component, where mutations don't trigger an update on the prop as a whole.
 			if (name === '$$props' || name === '$$restProps' || binding.kind === 'prop') {
-				serialized = b.call('$.deep_read', serialized);
+				serialized = b.call('$.deep_read_state', serialized);
 			}
 
 			sequence.push(serialized);

packages/svelte/src/internal/client/render.js

@@ -44,11 +44,11 @@ import {
 	push,
 	pop,
 	current_component_context,
-	deep_read,
 	get,
 	set,
 	is_signals_recorded,
-	inspect_fn
+	inspect_fn,
+	deep_read_state
 } from './runtime.js';
 import {
 	render_effect,
@@ -1950,7 +1950,7 @@ export function action(dom, action, value_fn) {
 			// This works in legacy mode because of mutable_source being updated as a whole, but when using $state
 			// together with actions and mutation, it wouldn't notice the change without a deep read.
 			if (needs_deep_read) {
-				deep_read(value);
+				deep_read_state(value);
 			}
 		} else {
 			untrack(() => (payload = action(dom)));
@@ -2858,10 +2858,12 @@ export function init() {
 	if (!callbacks) return;
 
 	// beforeUpdate
+	if (callbacks.b.length) {
 		pre_effect(() => {
 			observe_all(context);
 			callbacks.b.forEach(run);
 		});
+	}
 
 	// onMount (must run before afterUpdate)
 	user_effect(() => {
@@ -2876,10 +2878,12 @@ export function init() {
 	});
 
 	// afterUpdate
+	if (callbacks.a.length) {
 		user_effect(() => {
 			observe_all(context);
 			callbacks.a.forEach(run);
 		});
+	}
 }
 
 /**
@@ -2892,7 +2896,7 @@ function observe_all(context) {
 		for (const signal of context.d) get(signal);
 	}
 
-	deep_read(context.s);
+	deep_read_state(context.s);
 }
 
 /**

packages/svelte/src/internal/client/runtime.js

@@ -1244,6 +1244,29 @@ export function pop(component) {
 	return component || /** @type {T} */ ({});
 }
 
+/**
+ * Possibly traverse an object and read all its properties so that they're all reactive in case this is `$state`.
+ * Does only check first level of an object for performance reasons (heuristic should be good for 99% of all cases).
+ * @param {any} value
+ * @returns {void}
+ */
+export function deep_read_state(value) {
+	if (typeof value !== 'object' || !value || value instanceof EventTarget) {
+		return;
+	}
+
+	if (STATE_SYMBOL in value) {
+		deep_read(value);
+	} else if (!Array.isArray(value)) {
+		for (let key in value) {
+			const prop = value[key];
+			if (typeof prop === 'object' && prop && STATE_SYMBOL in prop) {
+				deep_read(prop);
+			}
+		}
+	}
+}
+
 /**
  * Deeply traverse an object and read all its properties
  * so that they're all reactive in case this is `$state`

packages/svelte/src/internal/index.js

@@ -18,7 +18,8 @@ export {
 	inspect,
 	unwrap,
 	freeze,
-	deep_read
+	deep_read,
+	deep_read_state
 } from './client/runtime.js';
 export * from './client/dev/ownership.js';
 export { await_block as await } from './client/dom/blocks/await.js';