For vector animation, as well as optimised bitmap animations Flash was blazing fast and GPU accelerated. Granted the plugin in general was bloated, and lots of Flash objects on a page slowed your browser down, but once it got going (for example in a full screen game or whatever) Flash was very efficient. I'd say the web platform has not yet caught up.

Flash is slow. In-browser animations using 'HTML5' (as in full cartoon animations) as well as SVG manipulations is quite a bit slower than Flash. Always has been. Will be for a bit longer.