In response to lucid's thread, I've made two projects to test HTML5 graphics performance, one for CC and one for C2. Both create as many sprites on the screen as possible before the framerate falls to 30fps. I've adjusted both projects to try and make them as fair as possible, they're attached at the bottom. This only tests the drawing speed - not the event engine efficiency.
The .cap is a Classic r1.2 project. Its engine is optimised C++ code and as you know it uses DirectX 9 to render everything hardware accelerated.
The .capx is a Construct 2 r45 project. Its performance depends almost entirely on the browser. IE9 is by far the best performing browser, so if you're on Vista or 7, try testing with that.
You can run your own tests to see how it fares for you. You will need to leave it running for a while, and give it at least 10 seconds to stabilise - it takes objects away if the FPS drops below 29, and adds more if it's still above 30, so it may take a while to settle down and get the best number. Make sure no other applications are running during the test, for fairness.
Remember: This is testing on-screen objects - many games will have hundreds of objects, but rarely have more than a hundred on-screen at once. When they're offscreen, obviously they take no drawing time at all.
Here are my results. You can check your graphics card by clicking the about icon in C2.
Graphics card: GeForce Go 7600 (old laptop graphics card)
Classic: 748 objects
C2 with IE9: 619 objects
C2 with Chrome 12: 124 objects (if you go to about:flags and turn on the experimental 'GPU accelerated canvas 2D', this goes up to 450 objects! I think this will be enabled by default in a future version of Chrome)
C2 with Firefox 5: They've blacklisted my graphics card driver from hardware acceleration, presumably because it's old. If I force it enabled (about:config - gfx.direct2d.force-enabled), I can get up to 440 objects. Without that it struggles to get over 20 objects but I think that's because my laptop is so old they've decided not to support it.
Conclusion: Support is a bit patchy and still in-the-works on Firefox 5 and Chrome. IE9 performs the best, though, and is 82% as fast as Classic. That's awesome, considering it's running in a browser! FF5 and Chrome, once their support is sorted out, are both about 60% as fast as Classic. That's still fairly good, considering most games in Classic probably don't push the renderer that hard.
Bottom line: HTML5 seems to be pretty fast!