Can't play videos smoothly with resolutions higher than 1920*1080

Hello, I’m a Hi9air user and I have this problem where videos with resolutions greater than FHD would lag and not play smoothly, specially if they’re 60fps. It’s weird for me since I remember a while ago I could easily playback a 4K video on it with no lag but at some point in time every high res video started to lag and have cut of audio. I use Mx player pro on hw+ settings.