Media player from demo gallery of Boot to Qt image for Raspberry Pi 3 has poor performance playing Qt_video_720p.webm (default video there).
In Boot to Qt images before 5.11 it was fine. It started with 5.11 and it is like that in 5.11.1 too.
Apparently, some media plugins are missing. Valentyn Doroshchuk can help to understand which ones exactly.