My 2c...
The speed of the machine and available memory, both are higher than client 1, so this may make a difference to page rendering, giving the impression of faster surfing.
Are all the network cards the same spec? ie, are they all 10/100?
We have NT, Linux and Mac machines all accessing the Internet via our e-smith box and there isn't a huge noticeable difference on any of them.... they're just all slow!! :^) We still have 56k modem dialup access here! :^(