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.