You could try to limit the FPS of the game. I think there should be an option in C:\Games\Starsector\starsector-core\data\config\settings.json that controls FPS.
Sadly for me no FPS limiter seems to work usually, but I have found a program that always works for me on any game. I use Dxtory to force FPS to stay at a specific rate. All you have to do is install it, go to the Advanced Tab and on the bottom check the Limit Video FPS tickbox and enter the FPS you want. It works without fail and obviously if you will put something like 15 FPS, the while game will become slower. I am not sure though now it will look for the game to run on 15 FPS. For that you will have to actually test it yourself.
You do NOT need to register and buy Dxtory, to just use the FPS limiter.