Solve the issue of diagnosing perf problems from a single hot function

Here is an example perf problem that involves a single hot function. I had to fix it by instrument the function itself with timing information.

This is a JS only example, it would be nice to have a C++ example.

From PR #695
Before: https://perfht.ml/2qs05TQ
After: https://perfht.ml/2qrczLn

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-705)