Considering that the CPU usage on a Mac is lower than the Flash version of the same demo, that perhaps says a lot about the prospects of Flash on mobile devices . . .
That says nothing, considering this is coded up in an interpreted language (JS), compared to the speed of a compiled language (C/ObjC) on an official iPhone version of Flash, should there ever be one.
Languages aren't everything. Gnash, written in C++, never succeeded in replacing Flash even on platforms like *BSD where Flash was nonexistent.
In particular, when dealing with Flash, feature-completeness is going to be much more important than perfect performance. Since the Flash VM on Linux and OS X is so slow and software-renders everything, it really won't take much to improve on it. The advantages of a dynamic language are probably going to far outweigh C++'s speed, and Javascript's similarity to Actionscript is icing.
Plus, Javascript can take advantage of whatever hardware-accelerated graphics library the browser vendor chooses.
