add comments to explain perf sensitive code

src/compiler/preprocess/index.ts

@@ -149,7 +149,8 @@ export default async function preprocess(
 			filename
 		});
 
-		if (processed) {
+		if (!processed) continue;
+
 		if (processed.dependencies) dependencies.push(...processed.dependencies);
 		source = processed.code;
 		if (processed.map) {
@@ -160,7 +161,6 @@ export default async function preprocess(
 			);
 		}
 	}
-	}
 
 	async function preprocess_tag_content(tag_name: 'style' | 'script', preprocessor: Preprocessor) {
 		const get_location = getLocator(source);

src/compiler/utils/string_with_sourcemap.ts

@@ -59,6 +59,8 @@ function merge_tables<T>(this_table: T[], other_table: T[]): [T[], number[], boo
 }
 
 function pushArray<T>(_this: T[], other: T[]) {
+	// We use push to mutate in place for memory and perf reasons
+	// We use the for loop instead of _this.push(...other) to avoid the JS engine's function argument limit (65,535 in JavascriptCore)
 	for (let i = 0; i < other.length; i++) {
 		_this.push(other[i]);
 	}