Grafana k6
k6.ioPerformance
Script realistic load tests in JS/TS and find out where your API actually falls over, before your users do.
Other products from Performance
Free and open, and still the deepest way to see exactly why a page is slow — waterfalls and filmstrips, not vague scores.
The fastest way to get a page-speed report a non-engineer can actually read, though the good stuff now sits behind a paywall.
For teams who need to watch Core Web Vitals drift over time instead of running one-off checks — built on WebPageTest's guts.
The default answer to 'why is production slow' once a team is big enough to afford full-stack tracing across every service.