zridling: I was talking about efficiency, not security... but on the subject of security, while a lot of holes have been in java, flash, and the acrobat reader plugin, it's not like browsers themselves have been immune.
On performance, we'll have to wait and see. There's a lot of work going on wrt. optimizing JavaScript JITers, with a fair amount of competition between Chrome, FireFox and IE. But seeing how sluggish Panorama is in FF4 on lower-end machines, I'm not convinced "HTML5" is going to e better performing than flash any time soon...