Edit: to elaborate: do a simple simulated annealing on a 200x200 array in JS - it's measured in seconds on my laptop. I wrote a program in JS that did this (image segmentation, fg/bg seperation for recognition) and abandoned it because of speed issues.
I did some serious optimization when I wrote it. There was a "best practices" for V8 that shedded some light on the background of JS variable allocation, I followed it and got a serious boost. Also did lookup tables instead of runtime math with exp/log functions. But in the end: hit several worst case images that took more than 12s, which was a gap that couldn't further be reduced through optimization.
But you're absolutely right, I should redo it in PNaCl.