Around line #490 in utils.py
There's a function that is constantly updated, which checks if the active object is in a group, and does something.


Whatever that whole thing is supposed to do, if the active object has a group, it lags viewport fps to like 66%.



I'm using Blender 2.79;

I commented out the try/finally section and replaced the scene_update_guard variable with False (the end result to skip the whole check)