IT IS NOT YOUR CPU. I just retired my 10 year old pc with a 6400 Core 2 duo , 2 gigs of ram and a gt 610 1GB. I used to play blurays, mkv and avi videos at 1080p all the time. Everything was flawless in win 10 pro 64 bit, not so much in win 7 tho. Playback was a little choppy at the beginning, but it would be ok after 30 secs of so. I think you should optimize video settings and use(force) hardware acceleration on videos. Also make sure you dont have many processes running in the background