Game became too low on FPS sometimes

[Mithur]

After a Orc Schism come out,i desided that my 99 level hunter/necromancer hybrid character must blow the dust from his adventure gear and fight on.So i started new world(with huge world and raging hordes,dangerous monsters modifiers),and doin quest and saving folks,until i met orc from new Armageddon clan.After a while when i breaking through hordes of monsters to slay theyre evil master in one of many caverns,post pigeon flyes by with message that my town is under attack by Armageddon clan(game became twitchy but not too much).When i came back to town more monsters join assault,some orcs join as help to Armageddon,other monsters decided that my town is easy prey(while siege continues game jumps between too twitchy and normal).Fire spread here and there,but we slay invaders with out mercy(after siege fps return to normal).Today i continue adventure and find Eternal Darkness village,and after a few moments they start attack at my town.And things were too twitchy - unplayble.Is there any way to avoid that fps drops or what is it was?
P.S.You can see drops on video i recorded https://www.youtube.com/watch?v=6ryWZcWjsRM from 24 minutes till 33 minutes thats when Armageddon clan siege my town

[Lord Darkness]

Agreed. I'm also suffering with frame drops. Decided to reduce all settings to low while playing in 1080p, which often would give me a decent framerate (90+), but even then, on occasion, it drops below 60.

I noticed some effects like rain, storm and all seems to be heavy, but mainly it seems to happen often in high level games, probably due to the amount of people on clans, I'd guess.

It feels like it's not really stressing my GPU (GTX 1070), but perhaps my CPU (i7 3770), but I haven't really checked to confirm.

[Shadow]

I believe I have a save game from you so I'll look at it this week and see what I can do about the slowdowns.

Our games pretty much always stress the CPU and don't bother the GPU much.