Why is one client computer much slower than all the others?
The primary cause of network issues that we receive support calls about is a mismatch between the Ethernet speed and duplex negotiated by the switch and that of the Macintosh client. In some cases we find that it is actually the server that has the duplex mismatch. Either of these issues can be fixed by hard-coding the speed and duplex of the switch and Ethernet card.
Spanning Tree or 802.1d is one of the most common causes of speed and duplex mismatches. On most switches the default spanning-tree configuration will prevent the switch port from being activated until the switch can verify that there is no network loop on the port. This default behavior can be changed by enabling Portfast on Cisco switches or something similar on other switches. Further information about spanning tree can be found on websites listed in the related articles section.
Of course it is always possible that there is a physical network problem, so if the problem is limited to a specific computer or set of computers, you should investigate whether the affected machines are on the same switch or try swapping Ethernet cables.
If those standard troubleshooting techniques are not sufficent to solve the problem you may need to get a packet capture of the traffic between a client and the server.