perf: throttle markdown lexer during streaming

Throttle to 50ms while streaming so the lexer isn't re-parsing on every token.

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';
@@ -53,13 +54,38 @@
 		]
 	});
 
-	$: (async () => {
+	const LEXER_THROTTLE_MS = 50;
+	let lexerThrottleTimer;
+
+	function updateTokens() {
 		if (content) {
 			tokens = marked.lexer(
 				replaceTokens(processResponseContent(content), model?.name, $user?.name)
 			);
 		}
-	})();
+	}
+
+	$: if (content) {
+		if (done) {
+			// Final content, render now
+			clearTimeout(lexerThrottleTimer);
+			updateTokens();
+		} else {
+			// Throttle during streaming
+			if (!lexerThrottleTimer) {
+				updateTokens();
+			}
+			clearTimeout(lexerThrottleTimer);
+			lexerThrottleTimer = setTimeout(() => {
+				lexerThrottleTimer = null;
+				updateTokens();
+			}, LEXER_THROTTLE_MS);
+		}
+	}
+
+	onDestroy(() => {
+		clearTimeout(lexerThrottleTimer);
+	});
 </script>
 
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