Flash performance was always terrible on all platforms. It span up the CPU on my windows computer from the beginning and never stopped until its eventual demise.
That's from bad programming practices. It was very easy to write a terrible inefficient program with it since it was so accessible. There was nothing inherent in Flash that used a lot of CPU. The VM was pretty light weight. It also had only a software renderer for most of its lifetime which would naturally struggle with lots of crazy overlapping effects.
HTML5 video had significantly better performance in terms of CPU from the beginning, and I'm fairly certain the Flash video players weren't written by amateur programmers with bad programming practices.
