fix: adaptive throttle for markdown parsing during streaming

Fixes open-webui#20878

Implements adaptive throttling (50-300ms) based on actual render time:
- Starts at 50ms for smooth initial streaming
- Automatically backs off to 300ms if rendering is slow
- Returns to faster updates when load decreases

This provides smooth UX for short responses while protecting against
freezes during heavy tool call scenarios.

Author: Algorithm5838

src/lib/components/chat/Messages/Markdown.svelte

@@ -1,4 +1,5 @@
 <script>
+	import { onDestroy } from 'svelte';
 	import { marked } from 'marked';
 	import { replaceTokens, processResponseContent } from '$lib/utils';
 	import { user } from '$lib/stores';
@@ -49,13 +50,56 @@
 		extensions: [mentionExtension({ triggerChar: '@' }), mentionExtension({ triggerChar: '#' })]
 	});
 
-	$: (async () => {
-		if (content) {
-			tokens = marked.lexer(
-				replaceTokens(processResponseContent(content), model?.name, $user?.name)
-			);
+	// Adaptive throttle: starts fast, backs off if rendering is slow
+	let throttleId = null;
+	let pendingContent = null;
+	let throttleMs = 50;
+	const MIN_THROTTLE = 50;
+	const MAX_THROTTLE = 300;
+	const TARGET_RENDER_TIME = 30;
+
+	function parseMarkdown(text) {
+		return marked.lexer(replaceTokens(processResponseContent(text), model?.name, $user?.name));
+	}
+
+	function doUpdate() {
+		if (!pendingContent) return;
+
+		const start = performance.now();
+		tokens = parseMarkdown(pendingContent);
+		const renderTime = performance.now() - start;
+
+		// Adapt throttle based on render time
+		if (renderTime > TARGET_RENDER_TIME * 2) {
+			throttleMs = Math.min(throttleMs * 1.5, MAX_THROTTLE);
+		} else if (renderTime < TARGET_RENDER_TIME && throttleMs > MIN_THROTTLE) {
+			throttleMs = Math.max(throttleMs * 0.9, MIN_THROTTLE);
+		}
+
+		throttleId = null;
+	}
+
+	$: if (content) {
+		if (done) {
+			if (throttleId) {
+				clearTimeout(throttleId);
+				throttleId = null;
+			}
+			throttleMs = MIN_THROTTLE;
+			tokens = parseMarkdown(content);
+		} else {
+			pendingContent = content;
+			if (!throttleId) {
+				throttleId = setTimeout(doUpdate, throttleMs);
+			}
 		}
-	})();
+	}
+
+	onDestroy(() => {
+		if (throttleId) {
+			clearTimeout(throttleId);
+		}
+	});
 </script>
 
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