
Adobe and Microsoft know that there is no way that they can optimize Silverlight and Flash for every concievable device nearly as well as the browser vendors for those devices can. Android has a highly optimized browser that can offload functions to phone GPUs and optimize for arm, iOS has similar, and so on and so on. Every platform you can think of has a web browser that is being optimized for that platform. If Flash is more efficient than HTML5 on any platform right now, it is only due to a head start and that advantage will be gone within a matter of months.