I know, I'm sure many people have asked for mods which reduce lag, but I actually have a plan of how to make it work. This mod would be a plugin of sorts, which, when turned on, would turn off rendering on moons and planets that you didn't need. It would first check what SOI you were in, for example munar orbit, and then stop rendering anything that wasn't in that planetary system. By rendering, I mean terrain, terrain scatters, lighting effects, atmospheres, and even physics for the other planets and moons. If you were in orbit around The Mun, the only things rendered would be Kerbol, Kerbin, The Mun, and Minmus.
of course it could be customizeable, like you could turn moho on remotely if you wanted to see it's transit from eve.
I think this would really reduce lag. It might have already been done, so tell me if it is.