I've never had great wireless coverage down the far end of the house, and I'm fairly sure the performance of the Macbook Air wasn't a lot different after the upgrade.
Since then I had to send my Air away for an LCD replacement (done under insurance thank goodness - not a cheap exercise), and since it's come back, I've found that the wireless is terrible (=pretty much unusable) at the other end of the house.
It will connect to the wireless, but DNS resolution seems to take a long time.
I tried all the standard DNS cache clearing commands, but that didn't help.
When I do a ping to a site (eg stuff.co.nz), the times on the pings are ridiculously high (10-13000ms).
If I move close to the AP (a Fritzbox 7390 from Snap) the performance does seem better - although even last night I found the times on a ping to an NZ server would go all over the show (from 30ms up to 15000ms!).
I'm wondering if the WiFi antenna or system was disturbed during the LCD replacement?
Assuming for a minute that it's possible, then I have no idea how to chase this one (or to isolate/prove that the issue has been caused by the LCD replacement).