Airport Extreme Poor WiFi Performance

Question

I've been doing periodic bandwidth tests with speedtest.net for several years now. I have an account, so my results are saved. I do them from a Mac Mini and also from my iPad.
The MacMini is hardwired to an Airport Extreme. The iPad is, of course, using WiFi. Usually it is on the 5GHz network, but sometimes (I have no idea why) it selects the 2.4GHz network. The 5GHz network is "n only", the 2.4GHz network is b/g/n. Security is WPA/WPA Personal. The Airport Extreme is on firmware 7.6.1. (I rolled back in December, a couple of months after I noticed the degraded WiFi performance.)
Until mid-October of last year, the results on the iPad were similar to those on the MacMini.
For example:
On October 4 : iPad - 10.27 Mbps; Mini - 11.25 Mbps
On July 21   : iPad - 10.13 Mbps; Mini - 11.39 Mbps
However, beginning with tests done in late October, the results for the iPad are markedly inferior to those of the Mini:
On January 17: iPad - 00.49 Mbps; Mini - 06.14 Mbps
Today        : iPad - 00.87 Mbps; Mini - 08.69 Mbps
Oddly, when I switch the iPad to the 2.4GHz network, it does much better: 2.40 Mbps (today) But, still, much worse than before October and much worse than the Mac Mini. All the iPhones in the house are on the 2.4GHz network and get results similar to the iPad. However, none of them have as much test history saved.
I don't think it's a question of interference. None of my neighbors have 5GHz capable routers. On 2.4 GHz, I'm using channel 1. Both of my neighbors with routers are on channel 6. No cordless phones or other cordless devices in this house.
I have recently reset the network settings on both the iPad and iPhone.
So, what did I do to mess up the WiFi on my Airport Extreme?

Answer 1

Thanks.
Model is A1408.
I set the 2.4GHz network to channel 8.
I had to leave the 5GHz network to "Automatic" because choosing 36, 40 or 44 prevented both of my Airport Expresses from connecting. They would only connect with "Automatic".
All the names are alphanumeric and short: mgn, mgn5ghz, AirEx, etc.
I disconnected my MacMini's ethernet cable and just used WiFi. As expected, bandwidth decreased from the normal 10Mbps to about 2.8 Mbps.
I did not do a factory reset as family is about to come home and I don't want to be fiddling when they're expecting working WiFi. I'll do it tomorrow.
Here's the Wireless Diagnostics Log:
# --- Check Connectivity
# --- Wi-Fi Interface
          Interface Name                    : en1
          MAC Address                    : 00:24:36:f0:a9:33
          Network Name                    : mgn
          Active PHY Mode                    : 802.11n
          Security                    : WPA2 Personal
          SSID                    : mgn
          BSSID                    : b8:8d:12:63:32:8d
          Country                    : TH
          RSSI                    : -51 dBm
          Noise                    : -89 dBm
          Rate                    : 130 Mbps
          Channel                    : 8
          Channel Width                    : 20MHz
# --- Bluetooth
          Power State                    : On
          Paired Devices                    : 0
# --- Wireless Environment
          2.4 GHz Networks          : 2
          5 GHz Networks                    : 2
          Current Channel Networks          : 2
          Recommended 2.4GHz Channels          : [ 11 6 1 2 3 4 5 7 9 10 12 13 ] 8
          Recommended 5GHz Channels          : [ 36 40 44 48 153 157 161 165 ] 149
# --- Network Configuration
          Primary IPv4 Interface          : en1
          IPv4 Address                    : 192.168.0.76
          IPv4 Router                    : 192.168.0.1
          Primary IPv6 Interface          : None
          IPv6 Address                    : None
          IPv6 Router                    : None
          DNS Servers                    : 192.168.0.1
                                          8.8.8.8
                                          8.8.4.4
# --- Debug Log Entries
01/20/14 16:48:29.6470           Large preferred networks list count <31>
01/20/14 16:48:43.2710           SMTP port check time out
01/20/14 16:48:43.2760           Current Wi-Fi network is an AirPort network