Is it the encryption or the wireless connection itself or something else?
Yes, encryption does place some overhead on total throughput. But it shouldn't cut throughput in half. Furthermore, though you didn't tell us the details of your connection, even the slower 802.11b speeds (11Mbs nominal, 5-6Mbs real world) are significantly faster than most broadband connections, so the internet connection is often the limiting factor in download speed, not the wireless equipment.
If you want to test whether the encryption by itself is problematic, you could turn off encryption for a short while and see if the 2:1 download speed relationship persists between the wired desktop and the wireless laptop. If the laptop is still a dog, you know it wasn't the encryption.
Check your signal levels, etc. - a weak signal will cause slow transfer rates, with or without encryption. You may need to tweak the positioning of your equipment to improve signal strength.
What else is running on your laptop? Spyware/adware and P2P (even when you are not downloading anything yourself) are download speed killers. If your laptop is functioning in a fundamentally different manner than the desktop, of course the download rates could be vastly different, unrelated to hardware or encryption.