At the moment I'm using the MPV filter that comes with CCCP and AC3Filter.
The DScaler MPEG-2 Decoders and Cyberlink also are very fast. (DScaler may actually be faster - but it's so close as not to matter - I will check video quality more carefully when I get home, which my change decision)
I found FFDShow was significantly slower on my system (a VERY vanilla XP)