Background
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I have a 5th generation AirPort Time Capsule, bought refurbished from Apple about 6 months ago. It is on software version 7.7.3 and has worked without issue until recently.

Problem
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Two days ago the performance of my AirPort Time Capsule's wifi dropped dramatically. Whereas before I had no problem with AirPlay, or streaming videos stored on the Capsule over wifi, now videos buffer every few seconds, and AirPlay is chopped up to smithereens. Web browsing is also noticeably slower, and file transfer speeds to and from the device have fallen through the floor.

Diagnosis
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The problem seemed to start a day or so after I added an HP Envy 5530 printer to my network, though this could just be a coincidence.

To get an idea of exactly how slow the wifi had become, I put my Mac directly next to the Capsule and timed copying a 2GB (16000Mb) file to and from the device:

* To: 390s ≈ 41Mbps
* From: 527s ≈ 30Mbps

Both are less than a tenth of the theoretical maximum of 450Mbps reported when I alt-click the wifi icon in the top bar, so something is clearly amiss.

To isolate the wifi as the culprit, I connected my Mac to the Capsule over an Ethernet cable. I then saw that videos streamed without issue, files transferred at reasonable speed, and web browsing returned to normal.

To isolate environmental interference and/or my Mac's wifi client as the problem, I took the Capsule into the office and repeated the transfer using a 360MB file on a Windows laptop. The results were even worse than above:

* To: 88s ≈ 31Mbps
* From: 136s ≈ 20Mbps

Here is a screenshot from [iStumbler](https://istumbler.net/) taken at home while a slow file transfer was in progress. The top two networks are mine (5GHz and 2.4GHz) and it shows there is no channel contention. The transfer occurred around the middle 50% of the graph.

![enter image description here](https://i.stack.imgur.com/Ds7pX.png)

Remedy
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I tried a few things to fix the problem, but with no luck:

1. Ran Wireless Diagnostics but no problems were found.
2. Assigned the wifi channels manually to a few different random combinations.
3. Specified a different SSID for the 5GHz wifi and tried connecting through that.
4. Removed the HP software and deleted the printer from my Mac, and unplugged the printer from the wall.
5. Performed a factory reset of the Capsule.

Next steps
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Short of returning the unit to Apple, is there anything else I should try?