Nah, it'll still use the same % CPU but other apps will just get to skip ahead of it in line for the purposes of processing. This is actually a bad thing, and will negatively effect your game performance. Some games automatically start at low priority, though, so that you can multitask. That always severely annoys me because then the game lags more often. I mean, my web browsing is not THAT important hehe
But that's a good thought, and it would work just fine if its just going to sit at the login screen for a while. Just make sure you turn it back to Normal when you log in if you decide to do that because you're experiencing some kind of performance problem with other apps.