For mpv or VLC, enable hwdec in their settings:
Sometimes, installing the codec isn't enough. If you are playing a high-resolution HEVC file (like a 4K MKV movie) on older hardware, the video might play slowly or choppy because the CPU is doing all the work. You need hardware acceleration. install hevc codec for mkv video on linux mint link
Linux Mint provides a meta-package that includes the most common multimedia codecs, including those needed for HEVC and MKV playback. For mpv or VLC, enable hwdec in their