I have high cpu use in Ubuntu 19.04, and my screen seems laggy.
top I see gnome-shell running about 50% very often.
The new version of gnome-shell is not compatible with some extensions. In particular those trying to show systray, such as "TopIcons Plus" are not working well.
Disable extensions (one-by-one?) to see if this reduces CPU usage.
To do this, install the
Open tweaks, check the extensions tab on the left, and disable extensions on the right (shown below, POP_OS, but should be similar/identical in other flavors of Ubuntu):
An alternative to TopIcons Plus which seems to catch most of the systray devices is "KStatusNotifierItem/AppIndicator Support"