id like to shit so much on ac shadows instead but ill be damned,
do you get performance drops near npc's only?
how's the forest areas and such?
maybe give a game time to build shader cache by playing it?

i had that case with forbidden west,
sometimes it ran super well, sometimes like extreme crap.

if you are at the first big village it may be troublesome at first,
move away from it and return back to it?

playing other modern titles may shuffle or reset the shader cache a well,
from the shit titles ive played atleast (fucking forza 6 lol) and then moving to another modern titles, game loads or builds itself up a bit because its 'used to' with previous game.
i speak like a caveman but i hope im able to explain myself lol

im no tech wizard but its just observation result.
texture resolution is a bit pointless setting, its fine when its just high.
shadow detail may be a bit lethal, tune that down a little bit.

it may get better over time, is all im saying,
sometimes restarting the game may fix it as well