Fresh install, no mods. 32-bit on Steam.
When I first start the game, in any mode, it runs perfectly. I can launch pretty much any size ship (within reason) without issue. After a few minutes of playing, however, I notice the game starting to slow down. Within 5-10 minutes, the slowdown is significant even with just a lone capsule on the pad. Looking straight up into the sky seems to alleviate this somewhat at this point. Unfortunately, the performance loss doesn't stop there, and within 15 minutes the game is running at 1/10th speed or worse, and looking into the sky doesn't help. The complexity of the ship doesn't seem to make a difference. The same thing happens whether my ship has 2 parts or 2000.
I suspect it has to do with rendering terrain, because the slowdown is most noticeable when any part of the planet is in view. There is an obvious shift from the smooth framerate when looking up, to the game abruptly chugging as soon as the horizon comes in to view.