I have the same issue but only on some battles. Some work perfectly fine even though there is the maximum number of troops (450 in my case) engaged in active fighting, while during others if I have the fighting troops in my line of sight my game instantly drops to 1 FPS until I can manage to rotate my camera. I haven't managed to find what causes this difference.
I've also tried different settings (including setting Sound Quality to low), but none solved the issue. I also noticed heavy GPU usage when this happens.
Happens on both e1.0.0 and e1.0.1, and seems to be more frequent in campaign than in custom battles
i3-9100f
1060 3GB
16 GB DDR4 RAM
Game on Seagate HDD
EDIT : Just did some more tests, the issue seems to be way more present on the latest patch for me. For instanceI had a siege where I instantly dropped to something like 5 FPS when coming close to a group of around 50 idle soldiers. However another group on the other side of the map didn't cause many issues. Rotating the camera to hide the problematic group was not enough, I had to walk away (I suppose this loaded a lower detail LOD for the soldiers but I didn't notice it, also setting the Character Detail to Very Low did nothing) for my frames to go back to around 25 (which is still way lower than what I have usually, even in a siege). GPU was at 100% the entire time, but CPU and RAM were pretty low. Couldn't check disk often, but usage seemed also fine every time I tabbed out to check.
EDIT 2 : Just did another siege, on a similar map (snowy castle), and I had between 40 and 50 FPS even though I had an even bigger amount of soldiers on screen all charging toward the castle. GPU was still maxed out for some reason though.