What is causing slow Wi-Fi speeds on my Airport Extreme? I have a recent model (bought 1-2 years ago) Airport Extreme that, until yesterday, has worked flawlessly.

Starting yesterday, my wireless speeds have tanked. I first suspected my ISP, but that doesn't seem to be the problem - my speeds seem to be fine through my wired devices (albeit subjectively - I haven't measured), just not the ones connected via Wi-Fi.

My setup is:


A cable modem with a CAT-6 cable to the Airport Extreme.
A CAT-6 cable from the Airport Extreme to a cheap 8-port switch.
CAT-6 from the switch to a wall patch panel, which runs under the house to ports in 4 other rooms (where my wired devices live).


The devices that are wired (Apple TV, Amazon Fire TV, Xbox One) all seem to perform fine - I can stream video, download game updates, etc. at decent speeds. My Macbook Pro, Android phone, iPod and iPad are all affected by the slowness, though.

Thinking there was maybe some sort of priority given to streaming packets at some level, I also tried loading the Netflix player on my Macbook (connected via Wi-Fi), but that was also slow, leading me to believe that Wi-Fi, specifically, is the problem.

Some other pertinent details:


I have a 200Mbps plan via Time Warner, and a very recent cable modem (the model name/number escapes me) capable of close to 400Mbps. I've achieved the full speed of my plan regularly until yesterday. Nothing in my setup changed, so this almost certainly can't be a result of anything I've done.
Strangely, when I reset all of my internet hardware (modem, Airport Extreme, switch) and open a page in a browser on my Macbook, I briefly see fast Wi-Fi speeds. Within as little as a few seconds, though, my speeds drop severely (in the neighborhood of 0-5Mbps). I've even seen the same drop over the course of an online speed test - I can actually watch the "speedometer" needle gradually droop down to almost zero.
I need to try connecting my MBP straight to my cable modem to troubleshoot further - I'm sure that will tell me more - but the adapter cable I ordered is still in the mail.
I'll try a factory reset on the Airport Extreme tonight or tomorrow, but it would be nice to avoid that if possible.


Are there any common issues that can explain this? Any standard troubleshooting steps that I may not have considered?
1. Wireless interference
The most common cause of what you describe here is a common wireless
interference problem.
This probability is pretty high if your network loss of performance
was closely related with the move of wirelessly connected
computer or of pieces of furniture made of metal or glass (these cause
many radiofrequency reflections).
If you want to check this cause of your problem, download
iStumbler. Let it run for 10 minutes at the time when you have network problems. Then sort your neighbours network
by decreasing signal level. If your channel is the same one as one of your strongers neighbours, then change it for a channel which isn’t among their channels.
I have investigated very complex wireless interference problems since more
than 10 years with this exceptionnal quality software.
2. Use of the wrong network interface
The second most popular culprit for this kind of problem is the use
of the wrong network interface due to the infamous Automatic network
configuration.
To check pretty quickly if you are in this case, the easy method
is to make a new Network location (which in fact is not a location,
but a configuration) where the unique active interface will be
your wireless one (No BlueTooth, no Ethernet…). Make all the other
interfaces inactives (Make Service Inactive in the little dropdown menu).
The worst case I saw was a MacBook connected through the wireless network
but the one coming from the iPhone which was left configured as a hotspot.
I let you imagine the nightmare connection since the iPhone was fully
used… because the network on the MacBook was bad. In fact this was a 3G
phone connection overloaded…and shared.
3. Uninvited wireless scroungers
In urban environment, the third most popular culprit for your kind of
trouble is someone who either unwillingly or willingly connected to your
wireless network.
This is a potential problem if your wireless network is open, is using
one of the fake security functions named WEP or WPA, is using a weak
password like 123456 or password on WPA2.
To detect this, use first iStumbler once more.
And if you want to go further, check directly on your Extreme Base Station
who is connected on it. There is no better and clearer way to see these
scroungers.
Don't change your password at random as long as you didn't start a correct analysis. This won't help to understand a problem you might have
to face repeatedly as long as you don't see it and kill it. For example, if you are using a fake security function, a neighbour who was able to get your password in 6 seconds, will do the same with any password.