I get smooth to fairly smooth battles, depending on the number and complexity of ships. I use the Klingon ships a lot. Sometimes I will have 6-8 of the large cruisers in my fleet, along with a dozen or so smaller troop carriers in with them. The enemy fleets will get up to 13-15 ships as well. It will slow down a bit in the first rounds of those battles, but not too badly.
Overall, the gameplay is smooth with a good framerate. I get 70-75 in the shipyard, 30-59 or so (depending if I am using the mouse to move around or just sitting watching the ships float there) in the zoomed in map views, generally 25-40 in the combat viewer. Those are tested results, not guesses.
I normally play on the huge map with 6-8 opponents.
I just did a new test in the combat viewer.
6 of the large Klingon Negh'Var warships vs 19 Iconian ships.
The results varied because I was experimenting with the Throttle Framerate and AA setings.
Unchecking Throttle Framerate yeilded maybe 1-2 more fps overall.
The funny thing was with the AA setting.
I had it set for 2x, and was ending up with 18.5fps (1280x1024 res)
When I set it to 4x it actually went up to 25fps.
Guess I'll keep the 4x AA, since it both looks better and is a good deal faster.