I don’t think Flash is the one that requires a lot of computing power, it’s the fact of decompressing and decoding a very complex data in real time called “Digital Video”, Flash makes transparent the hardware calls implementation for the developers, with swf to html5 converter you will need to deal with conversion of flash to html5 players by rendering a lot of hardware engines, then multi-platform apps will be a true headache for the developers, the same problem than always: HTML hacks for each browser but with a more complex technology: HTML5