I've definitely noticed frame rate issues. Using a 3080, 1tb nvme ssd, 32g ram, and a Ryzen 7 3700x OC 4.5ghz. I keep my settings pretty dumbed down - I'm not a graphics snob, I just like building computers.
Sometimes, inexplicably, my fps TANKS for a minute or so. Like, 3-5 fps. Pretty much unplayably low. Hardware monitor doesn't indicate any significant load or temperature spikes during these (admittedly rare, but always involving a city or horde) occurrences. Game just goes full potato for a bit.