I joined some full 60 player official servers and the infected were moving at about 1/3 the speed they should be, based on what I've seen in Youtube videos. They are also stuttering instead of having a smooth animation. This makes them very easy to kill or I can escape them when they aggro by simply jogging away.

Edit: I can also confirm that when I join a low pop official server the infected move normally, fast and smooth.

Here is an example video:
https://www.twitch.tv/videos/348933652