> Also, this turns out to be an actual bottleneck way more often than people notice.

I wish more devs -- particularly web front-end devs -- tried out their work on something that isn't a top-of-the line developer computer. There are plenty of devices out there that are computationally constrained and/or memory-bounded. Moreover, no one ever runs these things in isolation. So, while profiling in isolation makes a ton of sense to rule out external factors, in the real world you're competing with all the other open tabs and windows. That's a tricky problem to model, of course, but it doesn't mean we shouldn't try.