By default, Desktop Effects or Desktop Compositing is enabled by default.

I have a VM in VirtualBox running KDE. When these desktop effects are enabled, everything is horribly slow.

This is how you disable them:

1. Open System Settings -> Display and Monitor -> Compositor
2. Uncheck “Enable compositor on startup”
3. Open System Settings -> Shortcuts -> Global Shortcuts -> KWin
4. remotve the SUspend Compositing shortcut

Restart KDE or the VM.