The mpv package (mpv-1:0.35.0-2-aarch64.pkg.tar.xz) seems to be broken on raspberry Pi 4. If you try to play a video with: “mpv -fs -vo=gpu VideoFile.mp4” the playback starts but it is only about 2 frames per second.
Downgrading the mpv - package fixes the problem:
pacman -U /var/cache/pacman/pkg/mpv-1:0.34.1-5-aarch64.pkg.tar.xz
Please fix the mpv - package, thanks 
We don’t maintain the mpv package. Arch Linux does. And the only differences between 0.34.1-5 and 0.35.0-2, is the upstream source update and a couple of added dependencies.
So it’s safe to assume that if something is broken since 0.34.1, it’s an upstream mpv issue.
Maybe check Issues · mpv-player/mpv · GitHub for similar problems?
Ok, thanks for the info. Any chance of switching back to the old package in the distro while waiting for mpv developers fixing the issue ?
You can install it from your cache and ignore the update. But it’s better to get it fixed upstream.