If QuickTime can play your video it will probably have the best decoding performance. If not, try mpv. I have the same Mac model and both of them can play 4k/60fps H264. I never tried 2.7k/120fps though.
I think I did a mistake shooting at 120 fps, turned out so many fps are used when doing a slow-motion video. Anyway, I heard iMovie can fix the issue is that correct?