Xcode suddenly becomes very slow

It sounds like you have predictive compilation turned on in the Xcode preferences.

First thing I'd look at is the HDD. Is it constantly being accessed? If so, it would indicate several possible things - memory shortage and lots of swapping, hard drive free space shortage cause it to hunt for space, heavy fragementation etc.

If thats not it, perhaps Xcode is running something in the background. Do you have any custom scripts you have added which could have done something?

have you rebooted?