Mob response time

When approaching hostile mobs it sometimes takes more than a second for them to respond to my presence. Specifically, when hitting zombies that were chasing me, they would be hit back and then would stop moving for close to a second. Also, when in the nether, wither skeletons would take close to two seconds before chasing me even though I was standing a block away from them. I'm curious if this issue is experienced by anyone else. Is this some kind of client lag? It may be an issue with my system. Please let me know.

Thanks for reading