Try these few more things, then I'm out of suggestions...

- Turn off CUDA / Mercury playback and just render our one clip.
- Turn on CUDA as per the link below

https://forums.creativecow.net/thread/3/959495

-The only other thing I can think of is a similar issue I had with an FX 6300 processor where the CPU cores were 'parked' resulting in massive slow down. Go to your Resource monitor and on the side by the CPU's you should see each core and whether they are parked or not. Core parking is used to shut down cores when they are not needed, looking at your CPU graph it looks like the CPU is not working efficiently.

You can try and disable core parking as detailed below to see if it helps, but make a note of the original settings so you can restore them. Also make a backup of your registry before you change it. Some AMD CPU's don't core park properly.

https://www.tenforums.com/drivers-har...d-10162-a.html

Other than that I'm out of suggestions, sorry.

edit - Oh and try a different codec as well.