I would advise that you manually select a channel for your WiFi 2.4GHz and leave 5GHz to auto.
Also depends on where you live...I live in a apartment building and lately my WiFi on 2.4GHz was starting to just be slow as hell all the time randomly on each device, since the iPhone / iPad you cannot tell when they are on 2.4 or 5GHz, its best to name the networks separately.
Today I finally gave my 5GHz network a new name and all my devices [Macbook Air, Macbook Retina, 5S, 5C, 5, 4] all are getting the same ping rates, and speeds.
I too have comcast and running a Apple Airport Express [Btw the router you have is amazing! -- the Asus diamond series are the absolute beasts of routers].
Before half of my devices would report slower speeds and now all my devices are reporting the correct speeds.
The problem was congestion in the airwaves, too many routers running the common channels [1, 6, 11] and the problem with setting channels in between the common channels is that some devices fail to work on them [despite supporting from factory --- one can only find this out by manually setting channels and testing each device].
My comcast speeds are 28-29/5-6 all the time
Also if you have a macbook with 802.11AC support then go ahead and find the "Wireless Diagnosis" app that built into OSX and walk around your home to see what channels other networks are running on, its a great tool and I have used it plenty of times!
use this: http://osxdaily.com/2013/12/11/find-best-wi-fi-channel-wireless-tool-mac/
NOTE: Once you have the wireless diagnosis open DO NOT CLICK CONTINUE -- INSTEAD go to "Window" on the top menu and click "Utilities" -- a new window will open, Click on "WiFi Scan" and that is way more useful in diagnosing the network and scanning surrounding networks!
