There may be a leak in either the rocketry preview or its most recent update.
I'm used to previews running much slower than non-preview branches. I understand it's due to debug code, and that makes sense. But there's a twist right now. When my game slows to a crawl, if I save, exit, reboot, and try again, I get most of my frames back.
Seems to happen gradually over several hours of play even on young & low-tech colonies.
Play a new game to cycle 150, take note of game FPS Reboot, reload the same save, and game's FPS will about triple.