The 2 gig limit is a hardcoded force dump when Sins by itself reaches 2 gigs of ram usage (See the TSOP topic for a detailed explanation).
The more AI's, Units, and Planets on the map. Then the more CPU power Sins will use to calculate ship/structure speeds, and positions, weapon hits, and damage, abilities cast, buffs, de-buff's etc. This is IMO the main cause of the late game lag in Sins. Thousands of units on the map at once, and pretty much only ONE core of a multicore CPU doing ALL of the work.
Ram, or more specifically lack of ram (less than 2 gigs) effect it too, but not near as much as it does CPU since most if not all of Sins assets load up at map startup, and stay loaded throughout the entire game. Ram mainly effects the textures, meshes, particles, UI, and sound. SoA 2 is designed to mimic, or do better than Sins as far as texture sizes, and poly counts (a lesson learned from the original SoA mod). If you have 4 gigs of ram, or more, and a decent vid card then you shouldnt slow down late game because of lack of ram.