there are a few different factors which can effect it; firstly the window sizes on an old OS, I am not sure how flexible they are even with a tcp optimiser. Secondly the transfer rate of the hdd and the bus speed on the motherboard. Thirdly, if the pc is that old I would be looking at the cpu as well.
The best thing to do is disconnect your pc from the router and switch and directly connect it to the shub in modem mode and see what sort of speed you can get. Speed tests arent always 100%, especially with the shub, so if you dont get the result you are looking for you want to be going to somewhere like
www.gamershell.com and downloading 5 or 6 large files and adding up their cumulative download speed. Better to do it during the day and not in the evening is as well because the network is always congested and will give you a false negative.