Today, my Gnome-shell suddenly started using 15-30% CPU constantly.
If I start a session as another user (a new one), the CPU use is normal (1-2%). I have tried to deactivate extensions, but I still have the same problem.
I am on Ubuntu 16.04 LTS.
I have emptied the list of apps to be launch when beginning a session. After doing that I restarted and all is ok now.