I find anything above 100ms is a bit too much for expert as bullets tend to just not do anything and zombies seemingly hit you even when they should be out of range.
However, yes, some players are... shocking to say the least. I don't even mind people who are new to the game and genuinely don't know how to play - at least they're trying. I know some (who are sadly on my Friends list) that have played the game for a long time, but are just plain bad at the game.
All it takes is a little common sense; such as crouch as much as possible - i.e. for shooting and when you're getting mobbed (so as other can easily shoot over your head and help), use the right click push as much as possible especially when reloading (but don't over do it and kill your stamina), try to keep near a wall or corner especially when a horde comes and just don't get in the line of fire - friendly fire is probably the biggest killer on expert.
Some just cannot understand this and decide to run off on their own and they just die, to which I'm glad as it means 1 less dumbass to cover