I'm seeing the exact same thing.
When I switch between source files using the project manager one core of the CPU may go to 100% and Xcode will freeze for anything from 5 seconds to about a minute. If I try to continue after this has happened a couple of times then Xcode will lock up completely with one core at 100% and I'll have to Force Quit.
In version 7.0 this was happening when I switched between Interface Builder and source code. In 7.1 it started happening when I switched between source code files, and now in 7.1.1 it's happening even more frequently - perhaps every third or fourth time I switch files - plus I've just had one occasion where Xcode locked up when I wasn't actively using it.
Xcode 7.1.1, OS X 10.10.5 on a late 2013 RMBP 15".