Changing themes fixed this for me on Ubuntu 17.04, as per #3 above (Settings->Appearance->Styles).
Radiance/Ambiance themes: high cpu usage.
All others: no problem.
Other suggestions did not work for me:
I don't have a ~/.config/gtk-3.0/*.css that was mentioned in #9.
I am already using a Notification Area, as mentioned in #7.
Changing themes fixed this for me on Ubuntu 17.04, as per #3 above (Settings- >Appearance- >Styles) .
Radiance/Ambiance themes: high cpu usage.
All others: no problem.
Other suggestions did not work for me: gtk-3.0/ *.css that was mentioned in #9.
I don't have a ~/.config/
I am already using a Notification Area, as mentioned in #7.