There's no magic answer here. We track various metrics (page load time, responsiveness, gc pause time, and so on) in telemetry. (You can poke around at telemetr

There's no magic answer here. We track various metrics (page load time, responsiveness, gc pause time, and so on) in telemetry. (You can poke around at telemetry.mozilla.org.) We have some page load benchmarks that load a recorded copy of various pages and track how long it takes to load. Sometimes capturing a profile of a particular site can help. Of the big JS benchmarks, we've found Speedometer to be the least bad, because there was at least an effort to mimic actual websites. Microbenchmarks are tricky because it's very easy to measure something other than what you intend: differences in inlining heuristics, say. In the long run, I expect performance-critical websites to migrate to webassembly.