I am going to assume that these are the sames sites -- that the sites in Windows run fine, and slow on linux.
I would say go over your network settings. You may have a problem with your DNS. Also, I would like to know what NIC you have installed. There may be a known issue with the driver. Or even a hardware conflict. I do know for a fact that my nForce2 Realtek NIC runs like ass compared to my Tulip based Linksys/Cisco. And the Linksys/Cisco runs like ass compared to my 3Com (3Com and the higher end NICS are with the money).