I have now played a few games....

I must be lucky; I host 2v2 games with large units, between 5000 to 10 000 denarii per army, and neither I nor my fellow opponent and allies faced any significant lag. At those denarri level, not all armies are fully manned (ie, with 20 units)
The only slow down is when I see flaming arrows. Also a lot of missile can be a problem (some slow down during a test with frobeastegg involving 20 archers units on each side ).
3v3 seems to be run OK too, although I have not tried a 3v3 10k large setting yet.

I got an Intel 3.0 Ghz, 1Gb RAM and a decent video card.

Louis,