This is not a new issue. We have been dealing with this for a long time. Pretty much since day one.
No there is no optimization mod for Rebellion. There really is no need for an optimization mod, because at maximum settings Rebellion doesn't use more than 1.7 gigs of ram (This is including the two DLC expansions). The dev's worked on reducing the texture footprints as much as possible, and fixed many glaring issues that were found during the creation of the Optimization Project for Trinity. Rebellion is about as "optimized" as it is gonna get. Short of re-writing the entire game. The textures are NOT the cause of the late game lag now. Its the CPU, or i should say lack of it.
The Iron Engine was designed for use in Single Core CPU machines running 32 bit windows XP. Vista was just being released when Sins was released. The reason for the late game lag is that Sins only uses a single core of your CPU. It does not matter what kind, How many cores, or how powerful your CPU is. Sins will use only ONE core. As long as your CPU is around the 2 ghz speed, or better range you should be ok for most of the game. Every one of us experience the late game lag.
Ill repeat what was said many times before. SD/IC WILL NOT update the game to use multi-core CPU's, or 64 bit. It will not be cost effective for them to do so. So the only things i can suggest if you are lagging that badly (before the usual late game lag) is play on lower graphic settings, smaller maps, with reduced fleet sizes, and stack your icons in the empire tree. Also turning off cargo ship icons seems to help too.
