HTML 2d Canvas upscaling - really inefficient

I started writing some test programs with the HTML canvas element. This is great, as you can actually write games in Javascript - efficiently - in principle.

My previous attempts have all used the DOM API which is not very convenient, and not very efficient.

I had assumed the canvas 2d drawing context was basically a software-renderer - it's not extremely efficient, but provided the canvas doesn't have too many pixels, you can still do a lot of work per frame on a modern machine.

Which is fine.

Suppose you have a canvas which is 640x320 pixels, you can then get it upscaled into whatever resolution is in the browser window, making the game appear the same size for everyone. Great.

Except, the upscaling in web browsers sucks performance. I tried Firefox 3.6 and Chrome 9. Both of them use loads of CPU scaling the canvas on to the screen.

If we use a canvas element without any scaling (no CSS width etc) then all is fine.

Scale it up to a large window, Boom! now it's slow as a pregnant snail. Bummer.

See Example here