gnome-shell 3.26.1 constantly uses 20-30% CPU

Solution 1:

It turned out that the gnome-shell process went down to below 10% when turning off the system monitor extension https://extensions.gnome.org/extension/120/system-monitor/.

It also stays below 10% when increasing the refresh time to 1sec, while it went up again as soon as I moved it below 1sec.

Solution 2:

I have come across the same gnome-shell cpu hog issue and it was kmailservice5 that fires up the cpu when you don't have Thunderbird. Whenever I click on a mailto link, the cpu goes crazy and so does gnome-shell and kmailservice5 does nothing. It's a reported KDE bug and has been fixed since but for other Ubuntu LTS like 16.10, the fix is to uninstall kmailservice5.