I have a PowerEdge 2850 server that used to work well under CentOS 5 i386. It was used with Asterisk as a telephony server, hooked up to the world via a T1 line.
I've since wiped it and installed CentOS 6 x86_64, and everything works well as long as you boot from the install media. As soon as you reboot from the installed image, things slow down to a crawl. It takes about a second for every letter typed on the local console to appear on screen. SSH access through the network is abysmally slow as well. Hard drive read speeds are around 0.5mbytes/s. Same thing happens on CentOS 6 i386. Updating the system using yum doesn't help.
A T1 line from a Cisco router is plugged into the T1/E1 card.