Its the game, not the rig.
Turn off cargo ship icons, minimize the fleet tree and I have heard that large screen sizes also contribute.
There is TSOP (The Sins Optimization Project) which fixes memory leaks, reduces poly counts on models with no quality loss and to many other thing to mention here but it helps a lot. It keeps the game as is.
The TSOP ad-ons also help.
Strike Craft reduction cuts the amount but increases the damage. Cargo Ship Reduction cuts the amount in half and doubles the load and a Pirate fix because raiding pirates returning to base will eventually have almost a thousand ships there. And a mine reduction is coming.