The code that accelerates the loading needs to be revised.
I'm lefting something behind that, on weaker machines, causes KSP >= 1.8 to stutter on the Main Screen. It's something related to the max FPS or something near it.
Currently, the workaround is to Open Settings and reset the Max FPS to Default once the Main Menu appears.. This will fix the performance problem (until the next game boot).
(but it's a pain in the ass)