Not a long time ago I updated Xcode to version 4. This new version spent a lot of time on indexing the project (it's quite large). That's why I would like to disable indexing. Searching through Xcode help and internet gave no results.
Open a terminal window and paste this command:
defaults write com.apple.dt.XCode IDEIndexDisable 1
You'll lose some features (autocomplete, jump to definition, some of the assistants won't work right). But you'll gain back ram and cpu.
For my project Xcode went from using 2 Gigs to a few hundred MB. (which I sorely needed to compile with ;))