Good, sbcl it's great for CL. And now with current CLX from QuickLisp (the one with daily releases, I can't remember it's name) MCClim runs snappy even under Intel n270 ATom machines. Under ECL it almost runs snappy, but the performance gain it's astronomical. From a really laggy UI to instant rendering.

EDIT: UltraLisp for QuickLisp.