We can explore a few options.....Tesserax has already mentioned some.....but the first thing comes to mind is that your 4th Gen AirPort Extreme is about 10 years old, and the average useful life of a router designed for home use is about 5 years. So, it is not unusual that the AirPort is slowing down. You have had very good luck with the product.
One thing that might help would be to reset the AirPort back to its original default settings and then set it up again to see if that makes any difference.
Another would be to replace the outboard power supply "brick" with a new power supply. See the link below for an example:
https://www.savingology.com/ProductDetails.asp?ProductCode=AIRPORT-EXTREME-MA073LL-A&click=2&gclid=EAIaIQobChMIk82-pu6g5AIViJ-fCh1FxgW3EAQYAiABEgKkq_D_BwE
If the seller agrees to take the power supply back if it does not help, your exposure is minimal.
There are no user serviceable parts inside the AirPort Extreme, so if a new power supply does not help to bring back some of the connection speeds, and the troubleshooting steps that we suggest do not help...... the AirPort is likely heading downhill and the speeds will continue to slow down further as this occurs.
I have an 8 year old AirPort Extreme here that was retired from service about 3 years ago, and I set it up again to see how it was performing. The AirPort would deliver 130 Mbps at 2.4 GHz when it was new, but now it does about 60-65 Mbps. At 5 GHz, the AirPort would originally do about 300 Mbps, but that was down to 160-170 Mbps when I performed a few quick checks.
I have not replaced the outboard power supply "brick" on this device though, so do not know whether that might help to bring back a bit more speed.
