The normal process for forwarding a port is:
1. Set up a Static IP address.
2. Forward a port (which you'd need to do for the Airport.)
3. Force IP within LW: Force IP instructions & sample image & 2 posts after that. UPnP is for devices that understand UPnP technology. If your devices do then forwarding a port would not be necessary. But yes Airport does use NAT firewall.

After LW's been on for at least 30 mins, do this test: Bug Report instructions & sample image Is it True/False? If True then that's good!

When downloading try to choose those files that have multiple star quality, 4 stars the best. And also try to choose those with many sources (ie: multiple no. under the # column.

If a file slows down to 0 or 1 KB/s for a significant period, select it from the Download window & press the Stop button. Then reselect it from the search results & press download. This can often re-vitalise the download. Back that up with the occasional new search if the process drags on. Sometimes LW needs some baby-sitting.