Maniac
Apolyton Sage
Any graphic on the map. The game renders every single tile weather you can see it or not. Terrain takes the bulk of the time due to the fact that civ isn't smart enough to realize if it's changing one tile or many. When a terrain (and some features too) changes, the game doesn't just redraw the tile it's on - it redraws all the surrounding tiles too.
Thanks! Do you also know if activating animations takes less time than changing graphics? The thing is, for Planetfall GeoModder has allowed some terrain improvements to change graphics when they are worked, not by adding animations but by swapping the whole graphic. AFAIK the AI removes and then readds all of its workers in all of its bases during its turn, which would cause a slowdown if swapping graphics takes longer than toglling animations.