Some users – especially those on Mac OS X and Linux – have complained about the relatively high CPU Usage of Flash for video playback.This was partially because the Flash plugin did not use the GPU to render video. Adobe has responded to some of those criticisms in the 10.1 and 10.2 releases of the Flash plugin by offloading H.264 video decoding to dedicated hardware and by introducing a new video API called Stage Video. In addition, the use of the newer ActionScript 3.0 inside Flash movies instead of the older ActionScript 2.0 improves code execution speed by a factor of around 10. But older websites that use ActionScript 2.0 will not benefit from this The software routines written by developers can also affect the performance of applications built in Flash, reasons that would affect HTML5 animations as well.
An HTML vs Flash comparison in 2010, listed non-Flash technologies as being slower than Flash when used for non-video animations,but in January 2012 some of them were faster.