I've been delving into networking a bit more, and have discovered all sorts of madness.
Seriously, the two worse things in the world are doing taxes and routers.
I'm still on an ancient ADLS1 modem router - why? Because it WORKS.
There appears to be no rhyme or reason sometimes, why there are wi-fi issues. People like to think they have it all worked out because they make one change and it's 'fixed', but there's an entire range of what can be wrong.
For example, I tried a newer Linksys ADLS2 modem router. Set it up to the best of my knowledge - making sure all the settings (which by the way, are the most cryptic things ever - VCI, VC-Mux, what?), and found it to be constantly dropping. What was the issue? Well, apparently, Apple devices don't like 40Hz channels on the single 2.4GHz band. Ok, so I switch it to 20HZ. Everyting seems fine.
Then all of a sudden, a few days later, my iPad gets excruciatingly slow. Everything else is fine. After hours of research, what is the problem? Apparently, it's not talking to the DHCP, thus assigning itself an IP not within the range for the router. But why slow? Why not stop working entirely? Why after several days? Who the Hell knows. Routers are the devil.
So what is the likely problem? Well, research showed that it was pretty common for linksys routers to do this, and it has something to do with the encryption on WPA/WPA2. More specifically, the iPad probably doesn't like the way Linksys routers do AES. TKIP would probably be fine, except the router itself doesn't have aTKIP only option, even on WPA1 (WPA2 is AES only).
So it was back to my ADSL1 router.
SO yeah, slow MacBook air?
Check:
if you're on a 2,4GHZ band that you're using 20Hz, not 40Hz.
Check that your encryption WPA2 is solid - i.e. see if you get better speeds by removing wireless security altogether nd work up from there (WEP->WPA(TKIP)->WPA2)
See if mixed modes, for example on encryption or 802.11b/g/n is causing issues - many a time, the mixed modes freak out when lots of devices use different things.
Also, check your DNS settings. I found that my older router hates having to do DNS work, and I lost a bunch of features, plus got incorrect routing when the router handled it. I've resorted to just adding my DNS straight to every device.