Not sure what to put here to reproduce it however I have found that this seems to occur generally when your character is running though i have found it has occurred while turning your character around while standing in placeI have noticed a couple of things when these FPS drops occur.
1. FPS drop while running
2. Running while in towns and villages which have infected nearby
3. Rarely occur when running out in wilderness.
Uncertain whether nearby players contribute to FPS drops.