Bad system performance while WLAN connection to DSL-Router

[Asetek]

The speed of my TP is horrible when I open .txt or .rar files (sometimes delay up to 30 sec), only when WLAN connection is active.
My notebook is connected with the router by WLAN and my desktop computer with LAN. Both systems use bitdefender firewall while XP firewall is turned off.
When I turn off WLAN on my TP with access connections, then there is no delay and a quite good performance.
The same happens if I shutdown desktop pc or turn off the desktops firewall. The drivers of my TP are up to date.

Is this a known problem and will it caused by the Intel wireless driver?

[padpad]

Same here!

It took me about 20 sec to open a pdf file, while I was downloading at 300+ KB through WLAN.

Is it because of high CPU usage? I'm not quite sure.