I7-9700K(OC'd to 4.6ghz/Watercooled), 32gb ddr, 2070 Super, Samsung 1tb SSD, MAX graphics settings on 3440x1440 resolution
Ive done a 600vs500 battle and saw no drop in fps.
I noticed a decrease on sieges with more than 300 troops by about 15-40fps. I run 75mhz refresh rate with 74.98 fps cap and get that throughout the game until sieges happen then it can drop all the way to 35fps. Doesnt seem to be the entire siege though. Seems to happen when the siege first loads up. I also noticed a difference(for the better) if i turn delegation off and command my own troops.
Seems the calculations the AI has to run for every troop is a bit too much currently. Try turning your delegations off to see if that improves it(F6).
You can tell things are messed up anyways in the scripting and pathing just by being in a siege andwatching your troops and the enemy troops be complete idiots and just run into the walls. Sometimes they will use ladders on rare ocassions.