The static objects, like systems, planets, bases, and that kind of stuff does not affect lag.
What affect lag are the moving objects, and the IA. The IA is calculated on the server, so everybody sees the NPC doing the same. The moving objects are calculated on the server too. That is what I can see through the game behavior, and is the most logical course of action to take. If IA were calculated client side, then two players in the same place would see NPC doing different things. Take as example the starcraft game recordings. Every time you play a recorded game, it ends different. Because it only saves seed info, and leaves the IA to do the rest.
(If you find any mistake in my English, please let me know via a PM)
(Really, I speak terrible English, so please, tell me if I make mistakes. I'd like to improve it a bit )