I'd like to see the amount of time the regex engine spent processing my test regex on the test text. it doesn't need to be anything precise, just a means of comparing the performance of one regular expression in relation to another. Perhaps it could be displayed in the yellow bar in bold.

Something like this,

Execution Time : (~12ms for 100 runs)

This is generally a bad idea because comparing regexes against each other is more difficult than most people realize. You have to account for various lengths of subject text that match, don't match, and nearly match. Also, internal regex engine optimizations can change between browsers and browser versions. I'm not ruling out a feature like this, but it sounds like a footgun that would mostly be used to spread misinformation about regex performance.