Considering all are still using what amounts to a modded Froyo kernel, the base systems all tend to run similar. The launcher you pick will decide how smooth or fast the basic interface feels.
This isn't Windows where you have WinXp, Vista and Win7 all running different speeds. It's more like all are running Win7, and you add a few tweaks and a different theme. Yes, you can make a difference, but for the most part, it's all going to run the same. Some tweaks can make certain things run better, but others slower.
The launcher is the thing that is most obvious, but it's a separate program from the rom. To see the differences between actual roms in most cases, you need a benchmark program, and test after test has shown there isn't much difference, unless you use Quadrant, where it gives you a false reading because of Stagefright crashing. We used to use Linpack because it was the most consistent (using an average of 3 runs).