There is no objective way of meisuring. Even benchmarks can not convey how the room will "feel" under every day usage.
Personally I switched very early from stock Vodofone GB to official ICS, then CM, then GB and now Slim room.
CM is fluid but slows down sometimes. Additional functionality is nice but I personally experienced slow wakeups and responsiveness.
ICS custom rooms are a bit laggy - some small hickups but imo faster than CM.
GB - most will agree that GB performs the best. There was a guy reporting that he couldn't play Temple Run on his CM10 but his buddy with stock Vodafone GB could play it fluidly. However you will have older desgin and apps (GMail, Maps, etc.).
It certainly is a bit of a pain in the ass to backup and reinstall and test and wipe cache etc. But that's the only thing you can do. For me slim room is good enough right now. I'm tempted to try chinese GB though but will probably wait for another CM build by Daz.
Lastly, I can't help but +1 on bathrooms. They give a special kind of privacy and yet are often used by others as well. Although I strongly suggest to install windows into the room. Fans are not enough.