[CentOS] slow responding firewall server

By any change do you have a speed/duplex mismatch?  What is the output of 
mii-tool <device name> or ethtool <device name>?

Do you have a hub or a switch?  What speeds does it support?

You can also do point to point bandwidth testing with Iperf 
<http://dast.nlanr.net/Projects/Iperf/>

Hope this helps.

Barry