Airport Extreme suffering wildly inconsistent speeds between 2.4Ghz and 5Ghz I have an Airport Extreme (will find actual version number later, as it's not able to tell from Airport Utility any more) that runs a 2.4 and 5 ghz network.  The 5Ghz one is almost always fast, but the 2.4Ghz one can produce some woeful speeds.  The following screenshots showing me using my ISPs speedtest tool from the same iMac on each network:





I'm simply at a loss as to how to start troubleshooting this.  It usually works fine, but sometimes is so bad I cannot even connect to it.  The faster connection is pretty close to my maximum, so I'm happy with that - if I plug in direct via a cable it's usually similar to this.

It doesn't matter how far from the router I am, if it's decided to be slow, there is nothing I can see to either fix or troubleshoot it.  The latest 7.6.1 firmware is on the AE, and it does not show any errors or warnings.
You are suffering from network congestion. Channel 11 is a very common channel and most 2.4GHz routers ship with that channel default. There are simply too many routers using channel 11 around your house. 

However, not many people in your neighbourhood are using 5GHz channels, so that is free from network congestion.

The solution to this problem will be to change your 2.4GHz channel from 11 to something else.