Even after the Dec 20 update on stable branch official servers I still have slow stuttering infected and actions like door opening have a 1 second delay. This happens on high/full pop servers and I've tried several. On low pop servers the infected work normally and there is no noticeable delay. From what I'm aware this is due to poor server performance. My ping was generally about 90 - 120 but this even happened on server with 50 ping.

Oddly when I joined an experimental high pop server the infected seemed to move fast and smooth except it did crawl through walls.