Created attachment 112433 [details] File with garbage in it I upgraded my main machine to Kubuntu 18.04 and was greeted with inexplicable slowness, lag, and application choppiness. Animations dropped frames, scrolling was laggy, and cursor movement seemed low-resolution. However, CPU usage was normal and KWin reported 40 FPS. In the end, the problem was found in the entry for my pinned virtualbox launcher in ~/.config/plasma-org.kde.plasma.desktop-appletsrc, which was 9600 characters long. Removing the pinned launcher fixed the issue. This is mostly a downstream bug (https://bugs.launchpad.net/ubuntu/+source/virtualbox/+bug/1769417), but having an excessively long entry in that file shouldn't slow down the whole desktop. Steps to Reproduce: 1. Install Kubuntu 18.04with Plasma 5.12.4 2. Install Virtualbox 3. Open Virtualbox 4. Right-click on its Task Manager entry and pin it (I'm using an Icons-Only Task Manager, if that makes a difference) Garbage has been written to ~/.config/plasma-org.kde.plasma.desktop-appletsrc (attached, see line 98). When the file has garbage in it like this, all of Plasma starts to feel slow and laggy. If I unpin the icon and thereby remove the garbage line, Plasma feels snappy once more.
Can't repo anymore; I think this was a matter of coincidence and pointer input issues with libinput from a buggy git master master version that I was using at the time.