Some computers lag on the Network

For some time now I have been noticing that some computers on the network are slow viewing web sites and some are very fast reguardless of the CPU speed. I can hardwire the computers to the network and still same as being on the wireless. But some comupters with the same specs are a 100 times more faster on the exact same network line that I had the slow computer on. I have noticed that some of the older Mac computers are really the ones that are having the trouble viewing websites fast and some are not. I do use the same network line for testing that is a Gig line to make sure its not the network line. I have even connected the computers direct to the cable modem to bypass local network and still the same results. I have even formatted the hard drives and reloaded some of them to see if it would make a difference and I get the same results. I know that these computers that are slow have been very fast viewing websites before. Also when I go to local network share locations and transfer files they go FAST all the time. I was wondering if anyone could help with this problem. I was thinking maybe its the MTU settings. the MTU settings on the Mac computers is set to 1500.

If anyone could help me please respond to this post ASAP.

THANKS!!!!

Edited December 19, 201015 yr by Deploysrs