The last months I noticed a very annoying behavior of Xcode 4.2 which results in responding very slowly to user interactions, e.g. editing code, debugging refactoring … mostly all IDE day to day work were affected.

It turns out that the problem is related to the project “workspace ” file. If  you delete em, .. yes you can :-), you get back a more or less speedy XCode.

The Credits for this valuable hint goes to link

have fun