router: URL-route matching is expensive

[lucab]

After profiling a Fresh application in production, we found out that the current URL/route matching through URLPattern is expensive both in consumed CPU time and memory usage:

fresh/src/server/router.ts

Lines 146 to 152 in d391924

	const processedRoutes: InternalRoute<T>[] = [];
	processRoutes(processedRoutes, internalRoutes, "internal");
	processRoutes(processedRoutes, staticRoutes, "static");
	processRoutes(processedRoutes, routes, "route");
	return (url: string) => {
	for (const route of processedRoutes) {
	const res = route.pattern.exec(url);

At the time of this writing our app has ~800 static-files routes, and a total of ~900 routes overall. Each one of those translates to a dedicated URLPattern instance.
On each request, URLPattern.exec(url) is called in a loop until a matching route is found (if any).
Unfortunately at the moment the URLPattern implementation in Deno is known to have some performance issues: denoland/deno#19861.

Under sustained organic traffic, it looks like this quickly becomes an hot tight-loop, consuming most of the CPU active time.
From the same profile, it looks like this is also causing a lot of pressure on V8 memory handler.

[lucab]

For future reference, this would likely benefit from the proposed URLPatternList: denoland/deno#20899

[marvinhagemeister]

Yup, I've discovered this yesterday too while inspecting a site with many static files. I'm already working on a fix.