Since 15.04 - 15.10 (both has the same issue) using Pepper Flash with Chrome or Firefox leads to CPU overload. Firefox is more problematic as it does not thread things, thus leading to very laggy computer. Chrome handles it "better" by threading and I can at least watch a youtube video without it being too slow. But in both cases this is not a good solution.
I wonder though if it is not H264 that is the problem. Indeed, when watching a Youtube video at 360p, the issue does not appear, only when looking at 720p or 1080p videos does this leads to a problem.
I could not find anything online about it.
My first guess would be: 1) PepperFlash issue 2) H264 library issue 3) Graphic card not taking up the stream to decode and Ubuntu sending it all to the CPUs (but a raspberry pi can decode 1080p vids ! I have an i7 O.o !)