I've tried all the major ROMs out there and it looks to me like Wes Garner's "Build" v10.2 ROM is the fastest one available today. I tried the base version with CFS (Completely Fair Scheduler) and the 10MB RAM hack.
He bases his ROMs on Cyanogen Mod, then tweaks them some more.
Garner adds in lots of goodies, like additional packages for the very latest Facebook, the hacked Google Maps with turn-by-turn GPS Navigation for outside the US, the Advanced launcher (based on the Hero UI) and lots of other stuff.
Check it out at either AndroidSpin.com or xda-developers.com in the Dream dev forum. The ROM supports both the HTC Dream (G1) and Sapphire (HTC Magic / MyTouch3G).