18 is a lot to have on one Wireless segment (the actual throughput speed on 802.11b is around 5-7 Mbs, and it's very inefficient), it would cause slowdowns although I can't imagine it would cause a 20 minute login time, I think TeeDub's on the right track.

If you do use a second AP, make sure it's on a different channel too.

I used to sell Wavelan/Orinoco wireless products, and I seem to recall that one of the benefits of their Access Points was that they could accept two WiLan cards, and effectively run two separate wifi networks and balance the load appropriately. Your particular scenario was used as an example particularly when selling into educational markets. Sorry, can't be more specific than that, memory isn't what it used to be, but might be worth looking in to?

Hope this helps...

KV