perf: don't use tracing overeager during dev

#17176 is a case where many sources are created and then written to (due to Svelte 4 prop mechanics), and our tracing kicked in eagerly. That combined with the excessive depth of the related stack traces slowed things down tremendously.

The fix is simple: Don't record stack traces until we've seen this source get updated for a couple of times. Additionally we now delete the `updates` map after a flush. Previously it was just an ever-growing stack trace map.

.changeset/salty-hounds-worry.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+perf: don't use tracing overeager during dev

packages/svelte/src/compiler/phases/types.d.ts

@@ -53,6 +53,7 @@ export interface Analysis {
 	/** @deprecated use `runes` from `state.js` instead */
 	runes: boolean;
 	immutable: boolean;
+	/** True if `$inspect.trace` is used */
 	tracing: boolean;
 	comments: AST.JSComment[];
 

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

@@ -608,6 +608,8 @@ function flush_effects() {
 	var was_updating_effect = is_updating_effect;
 	is_flushing = true;
 
+	var source_stacks = DEV ? new Set() : null;
+
 	try {
 		var flush_count = 0;
 		set_is_updating_effect(true);
@@ -643,12 +645,24 @@ function flush_effects() {
 
 			batch.process(queued_root_effects);
 			old_values.clear();
+
+			if (DEV) {
+				for (const source of batch.current.keys()) {
+					/** @type {Set<Source>} */ (source_stacks).add(source);
+				}
+			}
 		}
 	} finally {
 		is_flushing = false;
 		set_is_updating_effect(was_updating_effect);
 
 		last_scheduled_effect = null;
+
+		if (DEV) {
+			for (const source of /** @type {Set<Source>} */ (source_stacks)) {
+				source.updated = null;
+			}
+		}
 	}
 }
 

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

@@ -188,18 +188,26 @@ export function internal_set(source, value) {
 
 		if (DEV) {
 			if (tracing_mode_flag || active_effect !== null) {
-				const error = get_stack('updated at');
+				source.updated ??= new Map();
 
-				if (error !== null) {
-					source.updated ??= new Map();
-					let entry = source.updated.get(error.stack);
+				// For performance reasons, when not using $inspect.trace, we only start collecting stack traces
+				// after the same source has been updated more than 5 times in the same flush cycle.
+				const count = source.updated.get('')?.count ?? 0;
+				source.updated.set('', { error: /** @type {any} */ (null), count: count + 1 });
 
-					if (!entry) {
-						entry = { error, count: 0 };
-						source.updated.set(error.stack, entry);
-					}
+				if (tracing_mode_flag || count > 5) {
+					const error = get_stack('updated at');
+
+					if (error !== null) {
+						let entry = source.updated.get(error.stack);
 
-					entry.count++;
+						if (!entry) {
+							entry = { error, count: 0 };
+							source.updated.set(error.stack, entry);
+						}
+
+						entry.count++;
+					}
 				}
 			}
 

packages/svelte/src/internal/flags/index.js

@@ -1,5 +1,8 @@
+/** True if experimental.async=true */
 export let async_mode_flag = false;
+/** True if we're not certain that we only have Svelte 5 code in the compilation */
 export let legacy_mode_flag = false;
+/** True if $inspect.trace is used */
 export let tracing_mode_flag = false;
 
 export function enable_async_mode_flag() {