The suggestion from DrGonzo is just not true. Objects almost only affects CPU load. If you don't believe it go to a bigger town set you grafics to low so the GPU is not limiting and try the different settings. The higher the setting the lower the fps even when your GPU's workload is below 50% (see MSI afterburner etc.)
I can't understand why it's not "acknowledged" as it's really gamebreaking. Most important feature of the mod was to have this huge visual range of players.
Zombies ARE rendered until they are smaller than one pixel as it should be for players as well!
I don't want to play with 20 and less fps so I can't see the players instead :/
Even an i5@4,7GHz drops below 20fps! at very high. Can't imagine how to play on a lesser CPU