KSP is laggy because it's a ridiculously detailed physics simulation. This has nothing to do with Unity.
(And i say this as someone who hates Unity for the actual graphics handling bugs it has.)
(And i say this as someone who hates Unity for the actual graphics handling bugs it has.)
We're not talking about Python here, C# is a JIT compiled language not an interpreted one. The performance could be roughly comparable given the same code.
I think they actually just released a new version that supports a more newer runtime but I doubt KSP upgraded.