Main issue in KSP's case is non-multithreaded physics, because even Unity v4 uses very old version of PhysX. I can feel it even on my beast of PC, which has Core i7 3930K, two GTX 780 and 32GB ram. While Unity 5 actually updates a lot of stuff they've been using till now, including latest PhysX, which supports now multiple processors, multiple cores and multiple threads by default. And Unity 5 brings a lots of stuff. PhysX and IL2CPP is just two things out of everything planned by Unity Team for Unity 5. I believe Unity 5 has required capability to greatly reduce time developers need to optimize game, since lots of applications will be up-to-date by default.