Sounds like one or more of your DNS servers might be misconfigured. The machines try to query the first DNS server, wait a little and timeout, then hit the second or third server. Thus the delay. Check the settings with your ISP and verify you have 'em entered properly on your router/NAT box.
I've had 2 wireless laptops connect on my Linksys Router and 3 desktops connected, all 5 worked fine without any issues, AIM included.
Might want to check if there are any settings in your router configuration that might affect this, I doubt ports is the problem, maybe look into your Router's support site, there are many faqs/knowledgebase or even community forums for specific routers you can check on.
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