https://bugs.freedesktop.org/show_bug.cgi?id=91416
--- Comment #3 from Michel Dänzer michel@daenzer.net --- Without DRI3, you need to choose "Full scene repaints" in the kwin desktop effects advanced settings for "Tearing Prevention (VSync)". With DRI3, you can choose "Re-use screen content", which is more efficient.
You probably also want to enable "Suspend desktop effects for fullscreen windows".