In my current game on a smallest map I experienced a sudden massive slowdown, taking up to 10 seconds just to select a unit. I saved and restarted my computer but same problem. I went to an autosave from 2 turns before and was able to play for 5 turns then it happened again. I'm using Conquests, no mods, Athlon 2800, 1024 MB RAM and 200 GB of hard drive free. It's around 1830 and I've been cranking out mechanized infantry for about 10 turns but I only have around 2 units per city plus 2 stacks of 30. I've certainly had more units in a game before.