Connection speed via VPN different on different computers?

I live in Middle East and have a VPN connection to a UK server. If I connect using 3 of my 6 PC’s at home (some run XP some Vista and all networked)all is OK and a speed check shows around 1500mbps connection. The other three PC’s show around half that speed. They all connect via the same network, I’ve check the network card settings and all are similar. Any ideas why some are slow and some fast? I have a feeling it’s a hardware issue but can’t see what.