>>6
That depends on the exact function you use to edit. some function can be used to translate to fast JavaScript, complex functions usually do not.
Simple editing can be done with canvas and slow JS pixel transforms, it would just take more time.
If you write optimized JS, you can write an editor with about the same order of magnitude speed for 90% of code of equivalent C app using typed arrays, tight loops and avoiding GC.
That of course doesn't mean a complete photoshop clone in JS is possible, if written it would be like running current photoshop on a 486 @ 25Mhz(several orders of magnitude slower) which is unacceptable for performance of desktop application.
The current problem is the Canvas is not optimized enough even in latest Firefox 11. I expect the demand for video/image editing will push canvas to faster speed.