It's not the splatters and gibs I think. It's before any zombie even attacks. Maybe there's some extra demanding GPU effects that could be toggle-able, but I'm not certain which.
I observed this as well with a 5070ti and 7950x3d. It happens in between rounds, I only noticed because frames dropped from stable 155-160 to below 100 which is noticable to me, the game seems a bit choppy for a few secs before the new wave spawns. I don't think it's noticable on the web as much though or with lower refresh rate monitors + mid/high end PC's it shouldn't be an issue either.