Have a question about optimization (Total War: Warhammer)

I'm new to the TW series, so no idea how it optimization. But specifically in this game, something incomprehensible to me. The game gave me max settings in the benchmark shows more than 60 frames, and in the first battle in the campaign for the Empire of the fps may drop to 18-20, if the field of view is a lot of groups, from a different angle to the normal fps. Prompt, who knows, it really is optimize this mediocre?
Config: Core i5 3450 3.1 GHz, GTX 1060 6 GB, 8 GB RAM, HDD Seagete 2 TB