The parent said it's 'not graphics' that's the problem. I agree that it's not really the graphics that are an issue in KSP, but most likely the physics engine.

Essentially, most (unity) games don't allow their users to create content, whereas the essential part of KSP is the creation and simulation of lots of bodies.

I've definitely experienced a substantial slowdown when many entities are on the screen at the same time.

I'm not sure if KSP makes use of the normal Unity physics engine, a third-party one, or something developed in-house.