So it seems that custom battle scenarios numbers are bugged? I set up 250 vs 250 siege for testing purpose and game then ignored unit limit from game settings (which is 150). When I changed scenario to 75 per team it runs smooth now. And I think my gpu shows me some bugged numbers, cause it have no impact on games.
So my problem was resolved somehow, and I didn't even know. Frames dropped when i was playing siege in campaign, and custom battles deceived me.
Maybe it was steam version, 4gb patch or maybe it was this post from reddit
User Nytkim says:
(links are in that posts btw)
You should uninstall every Visual Redist, even the newer ones and restart your pc. Then download them from microsoft website, 2013 and 2015, both x86 and x64 versions and install. They are small to download and install fast. Uninstalling them all doesn't hurt your pc.