Airport Extreme suffering wildly inconsistent speeds between 2.4Ghz and 5Ghz
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.