I've done a bit of testing, and I learned that my computer really cannot handle a map larger than standard. I can use a huge map on Continents barely, but I cannot use large or huge on Terra.
Another thing that dissapoints me greatly is that you cannot use tall maps! I can think of dozens of scenarios that would call for that. I tried genning a Terra map that is taller than wide, and you do get nasty, unusable artifacts to the north of where the map is 'square'.
Really, I do not understand the poor performance of the map. The graphics are not so much more detailed than Civ4:Col, but I could make gigantic maps there with no trouble. With Civ5 on a standard map the 'normal map' textures lag behind like 3 seconds and slowly cover each tile one by one, which is far uglier than any offsetting benefits could compensate for.
I'm not expecting a 2007 computer to have peak performance for today's games, but I really don't see where the performance hit could be occurring. Shadows? Certainly this could be greatly streamlined somehow.