TP-LINK Archer C7 slows down over time

After about a month of continuous use, I have to restart my Archer C7 (OpenWrt 18.06.1) because the Internet connection grinds to a halt, the web UI is slow and the latency over Wi-Fi is terrible.

As soon as I restart the router, the issue disappears for a long time until about a month later, when the issue re-occurs.

What could be happening here?

The “usual” things I would check first would be memory, flash, and /tmp/ usage

  • free
  • df -h

....and a list of running services