Slight problem. I have a very good GPU and CPU, don't have any background processes, yet KSP runs at 20 fps.

[Dogecat2]

What can you do? (Other than change the actual PC).

 

[Scarecrow71]

Well, for starters...

A list of mods and your PC specs would help.

[Papa_Joe]

Yes, a log and list of mods will help, but also the complexify of your ships in flight, how many vessels are in the game save in flight, and when exactly are you seeing the 20 frames per second...

KSP is not optimized for GPU, so the CPU sees a lot of loading.  the busier your game, the slower it will run.  the more mods you have, the slower it will run.  lots of variables here, so a clearer description of your conditions will also help.

I have a test bed for my mods, with 2 space stations in flight, in different orbits.  they are fairly complex, so my frame rates will drop when those stations are in focus.  I have a reasonably high-end CPU (Ryzen 7 3800x) with a reasonably high-end Graphics card (Nvidia 2080 Super with 8GB ram).  Performance issues come with the territory with KSP.  YMMV