I have a PC that's far more powerful than the recommended stats, and yet I would get framerate drops of around 20fps and back up every few seconds, not just in-game but on the title screen or in cutscenes. Switching to low detail mode and playing with vsync didn't help, which isn't surprising because this is obviously an issue with the game. After some research, I found that this works:

1. While in game, bring up the Task Manager with ctrl+alt+del and selecting it or right click on the Start Menu and select it
2. Click the Details tab
3. Find Cryptark.exe and right click it
4. Choose "Set affinity" from the list
5. Click the "CPU 0" box to deselect it and click "Ok"
6. Close Task Manager and resume the game

This fixed the problem entirely for me, but it doesn't seem to be permanent, as sometimes after closing and relaunching the game you will need to go through the steps again, while other times it runs fine in another session after doing this even though the "CPU 0" box goes back to being selected. Just run through the steps again if the issue resurfaces.

It should also be possible to set this to run in a batch file or in a program shortcut argument, but it does require entering the CPU core selection in hex, and I haven't tried it. Just search for "set program affinity" and you'll find instructions to do it in different ways.

--Credit for this fix goes to jorsen over on Reddit, who discovered it in Stardew Valley here:
reddit.com/r/VFIO/comments/mkll0y/tip_fix_stardew_valley_stutters_with_affinity

Because Cryptark is an XNA game, I looked for similar problems with other games using it (like Stardew), and figured a fix would work across different XNA programs, which this does.