One player with ~480 corvettes caused CPU lag.
I was in a game yesterday where one player saw fit to build about 480 corvettes, and it caused my CPU to lag mid-game. I normally don't have a problem until late game and then only when there are huge amounts of fleets around. The implication of this is that corvettes are either too good of a value or that they need to be modified in some sort of a way so that they are larger, hardier ships that consume 4x or 5x of the current fleet supply while having 4x or 5x of the current hit points and damage delivery. My rig:
Socket 939 dual core Opteron 185 overclocked to 2.8 Ghz.
3 GB RAM
8800 GT
Win XP Pro SP3
(I know, it's old, but it gets the job done for me.)