JSBen.ch – Performance benchmarking playground for JavaScript
jsben.ch
jsben.ch
edit: It's when the code throw some error. The user should receive an alert.
Benchmarks are A/B tests. What you are talking about is called a "profiler" and if you google "JavaScript Profiler" quite a few will come up (including an extension).
Modern Browsers have a "Task Manager" in "three dots" > More Tools > Task Manager.
I realized I don't know what the actual bottlenecks are on websites. Background processes adding up across tabs I assume? If someone knows please tell me/us.