Have you checked out SASS and the new libsass library? If you want speed, you can't beat a 0-dependency c / c++ library.
20 times the speed of JRuby based Less is not shabby though.
edit: Performance was never the top motivation though. Architectural concerns and resilience were. Although based on a decent JavaCC grammar generated parser DSS should be able to eat any CSS and spit it out again as-is. While I certainly wouldn't want to write such dismal CSS, I also don't want to work on some richtext editor CSS just because I want to integrate that with our system.