EDIT: I was seeing a Data Rate of 11 Mb/s for my MacBook Air 2011, even though my signal Quality was Excellent with a RSSI dB of about -32. When I have the Radio Mode: 802.11n only (2.4GHz) setting on, I see a Data Rate of 144 Mb/s, which is the highest achievable rate with any Apple AirPort device on 802.11n (2.4GHz).
I have an AirPort Express. Here are the troubleshooting tips that seem to work for me.
I tried doing this thinking since it was a new update, Apple probably changed how settings are saved to the AirPort so to make sure all settings are saved properly and to reduce or eliminate bugs, I suggest doing this to start things off:
Try restoring your Airport by opening the AirPort Utility, click on the AirPort device, then Base Station in the menu bar on the top of your desktop, and finally Restore Default Settings... This seems to have fixed some issues for me on my AirPort Express 802.11n.
Later I found that my MacBook would load very slowly or not even load webpages at all. I saw I was receiving a 11 Mb/s Data Rate like there was a 802.11b device connected even though my MacBook was the only wireless client connected to my AirPort. I just tried this and played a 5-minute 1080p YouTube video and it did not stutter once. Hopefully this sticks and is the fix we are all looking for:
If you can deal with having 802.11n only Wi-Fi, then click on your AirPort device in AirPort Utility, click Edit > Wireless > Wireless Options... > Radio Mode: > 802.11n only (2.4GHz) > Save > Update
I was already having wireless issues with the Motorola gateway Time Warner Cable gave us to use Road Runner with and I set up my AirPort to extend the network through ethernet to make a fix. Now my AirPort is having issues too, unless a RSSI db of -80 from my neighbors is interferring on the channel 6 that I use with it, but I find that highly unlikely seeing how a number of others are having issues as well. I hope the Time Capsule I am getting on Monday does not have similar issues. The 7.6.1 update definitely has some bugs.