You are right, our optimization workers will have a lot of work to do to optimize those images. In that case we recommend the use of "callback_url" option to eliminate the possibility of a request timeout.
I've just tried compressing a 12 meg png file with pngcrush locally (generated with `convert -size 1000x2000 xc: +noise Random noise.png`, it only took a few seconds and didn't tax the processor much. Maybe pngcrush handles it better than other things but that didn't seem onerous.