I have upgraded system recently and I can add something about mpv on unaccelerated hardware.
mpv can use different kind of video outputs.
Default is opengl. It works well for me with acceleration by Intel's Sandy Bridge GPU. But I have dual GPU system and without applying patches I don't have acceleration. If I don't have acceleration xv video output works reasonably good.
It seems that, sadly, vaapi video output is not available in OpenBSD' mpv. It is in Gnu/Linux distros.
Code:
## prints available video outputs
$ mpv -vo help
## to watch movie
$ mpv -vo xv /path/to/file.extension
Or by config file ~/.mpv/config