I have recently downloaded Xcode update 4.5.2 and it seems to performing slow. I have tried other applications and everything is running fine. I'm currently developing an app and it's been performing so slowly, that I can't edit an UIlabel box.
Just in case: remove the contents in the "~/Library/Developer/Xcode/DerivedData" - Folder and see if it still stalls. I had a sluggish Xcode once and that helped.
来源：https://stackoverflow.com/questions/13294521/xcode-4-5-2-working-issues