I noticed this happens only with people around me (close to the town), although there is still a perfomance drop without any other players. With players I get a slideshow of 5fps, without other players around I get playable 15 fps.
I have a gaming laptop (Asus G750)

This happened to me twice, the zombie ignores you if you don't approach him from the front. The zombie with this behavior was always stationary (not moving) until I circled around him and got agro in front of him