My point is more that I've known more than a couple of frontend developers who leap on nifty frontend profiling tools like these before they've even tried a couple of well-placed table indices, or asking the database developer or systems guy about them. In my experience it's a lot easier to wrest 500ms from a bad query than 5ms from some bad JavaScript.

In reality however you will usually wrest 50ms from DB and can shave off seconds in front-end. It's usually not about Javascript loops optimization, it's about optimization of resource loading and minimizing the effect of network latency. Even you if shaved 500ms of DB request, hardly anyone will notice if page waits 15seconds before it has everything fetched to be able to render.