.mkv is extremely heavy on the cpu, and even modern low clocked laptop cpu's will completely choke on 1080p .mkv
All that needs to be done is enable hardware acceleration on your media player settings which offloads everything to the gpu.
And your .mkv files will play just fine with 0 % cpu usage. Even crappy onboard intel HD gpu from 10 years ago will play it perfect.
Popular and mainstream media players like for example VLC come default installed with hardware acceleration disabled offloading everything on the cpu. Go to settings, enable hardware acceleration and you are good to go.