2D games are rendered the same way 3D games are, as polygons with textures on them, so increasing 3D performance improves 2D performance as well. They're really the same thing. The amount of polygons a card can push isn't the whole issue - there's a bunch of other things like pixel fill rate and shader performance to take into account too, all of which continue to improve at quite a fast rate.
The video card isn't the total issue, either, as things like physics can cause the CPU to be what holds up the next frame.
There's compilation time, where a game is being compiled in the background at start, so it can take a few seconds before the game is running optimally while it's compiling in the background. My current game requires about 10 seconds before the framerate gets smooth.
There's also the issue that chrome and node webkit are currently on version 32, which has performance problems on some pcs as it's reverting to software rendering when it shouldn't. Chrome when fullscreened has a framerate in single digits on my machine currently, but chrome 33 should fix it. By switching to windows 7, you're probably using different video card drivers, which might have affected the performance, or caused your card to now be blacklisted when it wasn't before.
Anyway, in my experience and testing, C2 actually rendered faster than CC, but it has been a while since I checked. Event performance I tested CC to be 1.5-3x faster than C2, but in general I don't need that speed (though I would certainly like it).
As for the other problems with previous versions of C2 being faster, I can't comment because I haven't noticed a difference, though that doesn't mean there hasn't been one - the game I'm currently working on doesn't have very many sprites on screen at once. I don't have nearly the most powerful computer either, it's an AMD 4400+ with an Nvidia 9800gt. Have you been able to isolate the issue with a .capx?