Why one of the wireless networks is very slow with my laptop?

I am trying to connect Toshiba R630 Laptop to one of the wireless networks in the office. We have three wireless network setup out of which one is proxy enabled to which I am able to connect but the connectivity is dead slow for me while other laptops just work fine. a) I have all rights to connect to the proxy. b) I tried the same network thru wired LAN it works fine. At the same time I am able to connect to the rest of the wireless networks available which are open networks and they work great. I couldn’t figure it out I need help