I have never had this issue until today.
For the past several attempts today. I add a line of code. hit save. and once I hit tab everything freezes up. Gives me the spinning rainbow circle, until I force quit the app.
It was happening on my machine, and I thought to maybe update all the available Xcode and OS udpates today, so I did, but it is still happening.
I am using a MacBook Pro (Retina, 13inch, Late 2013)
processor 2.4GHz Intel Core i5
OS X El Capitan Version 10.11.2