I hear a lot of people moaning about lag while playing KSP, but TBH I've not encountered any. Granted, I don't use really mindbuggeringly big ships or go on especially long missions, but what lag are you folks actually seeing? The only annoying delay I notice in play is a couple of seconds on the pad before launch, but that's not really what I'd call lag. Lag is when you're getting a delay between inputs and what you see on the screen. Generally for me everything is slippery.
Core 2 Duo E8400, 4GB RAM, and Nvidia 8500GT, so not exactly a beast.