Live browser performance detection

Hi. I'm curious if anyone tried (or rather wrote a library) to live detection of browser performance. It could be useful for choosing javascript functionalities relevant for user's hardware, in a way similar to choosing CSS relevant for his/her screen. I know something like this will worsen the performance / loading time, but I'm curious how much - maybe in some cases it will be worth of that? I also know, that testing one functionality doesn't reflect general performance, but maybe it will be good enought for common cases?