What service or library I can use to monitor user-side timing for AJAX calls and page loads?

I want to see how fast / slow are AJAX requests and page loads in users' browsers. For now I only was able to find Newrelic Browser, but it is a bit too expensive / enterprise for my needs. I see a lot of services for JS errors tracking, like Sentry, Trackjs, BugSnag etc. But not much for performance monitoring.
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