Hang on SC, I've been running BCM Gold with the specs below for a few weeks now at 1024 x 768, 32 bit everything, the AA on at 4X and not only am I getting an average of 60fps but I gotta tell you this game looks fantastic! But after I destroyed a few ground bases I had things slow to a crawl as well, but ONLY in tacops while in planet view mode (the planet I had just bombed). Not just zoomed in on a mission zone but ANYWHERE in planet view. However, as soon as I zoom back to space view or leave tacops altogether I'm back to running at normal speed.
I've seen this issue elsewhere on this board, someone mentioned that after they RANDOMed a planet they had massive slowdowns when viewing said planet in tacops. I can't find the thread anywhere but I remember it was the exact same symptoms as we are having here, it was after the rebuilding phase had begun and someone asked the question "could it be all this AI rebuilding?".
Now the rebuilding may not be the problem directly but something IS happening after buildings are destroyed on the surface. Besides the AI rebuilding I don't know what the game does down in the dark corners of it's code, for answers to that question we must defer to the expert: Derek, what say ye?