I use KDE Plasma 5.18.5 and sometime apps becomes very laggy, every input such as keystorke or mouse click takes several seconds to take effect. restarting the app solves the issue. It happens on Chrome, Slack, Konsole, zoom and more.

any idea what could be the problem or how can I debug this?

This could be an issue with some Desktop Effects, have a look here: viewtopic.php?f=63&t=167168

Thanks. Looks like I can also temporarily fix frozen app, by switching setting back and forth in Compositor.

Looks like the problem on that thread was solved by replacing the kernel?
Sounds a bit drastic, isn't it?
I'm using 5.6.19-200.fc31.x86_64
is there any other solution?
This is the default kernel that was shipped with Fedora 31. Didn't replace it.

You can have more than one kernel installed on a system and change the kernel you want to use on startup, nothing drastic in this.

If a kernel doesn't load modules or doesn't provide necessary modules changing the running kernel is often the only solution.

Resolved by reverting to the default theme