When zombies walk in hordes they are walking slowly when I'm up in the air but when I go down they budge out and run fast this is a bug that's supposed to be fixed

What happened was I was in Creative spawning a lot of zombies just a test when I switched to Survival I started running so I switched back to Creative and flew up when I was up the zombies were walking really slow they were lagging out when I go down they run fast like Baby Zombies and Pigman
What I expected was the zombies to walk normally.
What actually happened was the zombies walked really slow as if lagging out.
It's actually huge zombie lag but what I mean is it's not causing me to lag it's causing only the zombies to lag because there walking slowly