I first ran into this problem when fossapup64 was released. MPV was taking about 5x the processing power to decode video and my N3150 couldn't even play SD videos.
MPV 27.2 in bionicpup64 could play 1080i50 h.264 with YADIF 2x in software quite OK on the same hardware.
I experimented quite a lot to try and fix it, including proving that MPV 27.2 would work on fossapup64. But in the end I just carried on using bionicpup64 for media playback.
An issue on this has just been raised and answered on the MPV github page: https://github.com/mpv-player/mpv/issues/9790
Putting --vd-lavc-dr=no in the config file or on the command line fixes the problem. I have verified that this works for my N3150 on fossapup64.
Direct render is supposed to make decoding faster, but it makes it much slower on an Atom processor.
Now I can retire my copy of Bionicpup64