Zoom!!! makes your UI load faster by preventing garbage collection during the UI load cycle.
In particular, it fixes bad behavior incurred by Rock addons, but it'll benefit you even if you don't run Rock mods, as well.
In my own UI, this reduced UI reload times from a whopping 93 seconds to 32 seconds, shaving a full minute off of my load time.
Caveats: This won't work if you're on a FAT32 filesystem (Windows 95, Windows ME, badly configured Windows 2000/XP). Also, if you don't run many mods, you likely won't see a huge benefit from this. The more mods you run, the more benefit you should see.