I don't think it's tied to any one specific unit, at least not the construction of such a unit. If I start with units, then the lag is there from the start. If I start with no units, the lag takes a while to show up.
Yea my mods run slower then the vanillia version. This is becouse of the way I setup relations and create the cache to make them more aggresive. which means you have more countries sending more unitsin for the attack. which means lower defcon, which means more units being built, which means more units deployed. So there ends up being allot more going on in my mod which causes the game to run slower. And I have said this many times. I prefer a game where **** is happening instead of a huggy huggy world carebears.
If your starting the game with no units, to speed it up. My mod is probly not for you. The game is probly not going to run very good at the fastest speed. exspecialy during the first 2 or 3 years. Your probly going tobe better off staying at normal speed. Even slowing it down to very slow when you yourself are doing allot of military operations. While it may take longer, the gameplay will be smoother. Dont forget the save feature. This is not your mainstream RTS that you play a game in one hour or one night. You might speed a couple days, or weeks, or even months playing the same game. Exspecialy in sandbox mod. If you really want a single night of play, dont play sandbox, play one of the scenario's (or is it campaign's? I can never remember which type they are becouse BG has em backwards).