The strace files haven't given any clues to the problem. Video playback shouldn't use a lot of cpu power, especially if you have a graphics card installed. I have integrated intel graphics but I do have an nvidia card which I will risk installing in my machine and see if this makes a difference. Meanwhile I've built libmlt and shotcut without frei0r-plugin support. Can you try them. You need to change libmlt6-modules specifically.