Video Playback – Fix HD 1080p .mkv Videos Not Playing Smoothly

This is because GPUs tend to have 2D and 3D modes. Normally opening DX(games) will kick the GPU up from idle/2D into 3D mode chaning from min clocks to max clocks. But when you open a video player its seeing a 2D application and applying those clocks despite the already running 3D application.

Listing your video player as a "game" like here should hopefully resolve it. This link is close to waht you need to do (its not Optima here). How to force my laptop to use the discrete GPU?

You are going to find some slowdown on a machine running Windows 7 with only 2GB of RAM, a Pentium(R) Dual-Core T4500 @2.3Ghz and an Intel GMA4500M when running high-quality, heavily compressed video. The reason you're able to play normal .mkv files more smoothly is likely to do with the level of compression. This from Wikipedia:

In comparison to AVC, HEVC offers about double the data compression ratio at the same level of video quality, or substantially improved video quality at the same bit rate.

So because your HEVC files are more compressed, your computer is having to do a lot more work per frame to uncompress everything.

What I might suggest is uploading these videos to something like Google Drive and watch them from there, letting the cloud servers do the hard work of decompressing. It's maybe not the best possible solution, but it's at least one solid workaround, and it's worked for me when I had similar problems in the past.