I've noticed performance starts to drop when there are about 300+ parts. When there's 400 it's really getting laggy. When there's 500+ the lag is unbearable. And 500 parts really isn't that much, it sounds a lot but it's not. This includes all utilities, components, struts, etc etc. I started launching and building a large space station but I had to abandon the project because once I had docked together a number of modules it just lagged so much it wasn't fun to play that mission anymore. I have an Ivy Bridge 3770K running at 4.5Ghz and the main thread totally saturates out the working CPU core. I can't really throw any more CPU power at the game. The space station orbits Kerbin hoping for some performance improvements some date in the future