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

Solution 1:

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.

Solution 2:

If you've got a mac, you can see which channels are running on other nearby wifi networks. Just go to 'About This Mac' (apple menu) and click more info. Click 'System Report' and select Network - Wi- Fi from the left hand pane. In the interfaces section you'll your current network, then all other local wifi networks complete with channel and mode. Really handy tool for finding a free channel. 11 used a lot near me for 2.4Ghz and changing this made a real difference.