Too bad my shit phone hasn't had CM ported to it, so I can't try it out.
For example, Galaxy Ace and LG Optimus One were very popular, and they're getting even the latest version of Android for them with CM. And last I checked JB was very fast on them. I also have JB on my own HTC Legend, which was also one of the most locked down phones ever. I went through a lot of trouble, and even had it "bricked" (not final) while trying to unlock it. But now I run a JB CM10 rom on it pretty well.
There's some guides out there on how to do it, but would have to look around for something specific.
Of course that also means you have to have an unlocked bootloader as well or the above is kind of moot.
The kernel was released... But I don't think I have the patience to try porting something myself.