A few ideas for things to check.
Is this a new problem or have you always had this problem? Did this problem appear slowly as you added clients to the network?
Does the server have enough RAM?
Do the clients have enough RAM?
Does the server have enough swap space?
Do the clients have a local disk with a swap partition?
Are the server disks slow? New disks are screaming fast and they are really inexpensive.
Use wire shark to see if your network has a lot of errors such as duplicate packets and out of order packets. That might indicate a hardware problem somewhere.
http://wireshark.org/
Make sure that the network cards are using full duplex operation for maximum speed.
Check the log files on the server for any kind of repeating problem. Security problems can cause a lot of overhead. Hardware problems may or may not be listed or may be difficult to identify from the log files. Nevertheless they are always a good place to check for any problem diagnosis.
Try turning off one client at a time leaving the others running. Does the performance on the clients improve when one client is off? That machine may have a problem that is slowing down the network. Check the network configuration and the network card on that machine.
I'll add to this post if I think of anything else.