I'm sharing a recent experience with an ASUS RT-AC68U Gigabit WiFi Router, which someone may find helpful.
In a home network, Internet access had become slow and sometimes did not work at all. This started some time in the last four weeks (maybe😀). Restarting the router might or might not restore access. Within a minutes or an hour or two, the problems would return. The network became unusable and an older router installed as a temporary measure.
There many wireless devices, more than 20 (Chromecasts, phones, tablets, laptops, camera's, etc), on the network.
The router shows high CPU utilisation (80-90%) even when few devices were connected. Disabling the radios made little change to CPU utilisation. Resetting the router to the factory default configuration and restoring the configuration step by step did not resolve the CPU issue.
There are many posts at https://www.snbforums.com/ describing issues with high CPU utilisation on this and similar ASUS routers. This post was particularly informative: ASUS RT-AC68U Firmware version 188.8.131.52.385.20253. Apparently, many users began experiencing high CPU and other issues after upgrading to this firmware.
The router in question had this firmware installed (bingo!).
Downgrading the router to version 184.108.40.206.385.20252 has brought CPU utilisation from 80-90% down to 17-25%, and Internet access is now reliable.
Firmware for this router including older versions can be obtained here. Click the Show All button.
Posts in that forum suggest that ASUS has experienced high CPU issues with previous versions of firmware, sometimes where the issue had been resolved then comes back in a later update!.
On further questioning, the user remembered being prompted by the router to install new firmware. Things went bad since then.
Version 220.127.116.11.385.20253 (2020/03/10) (high CPU) was to address the CVE-2019-15126 (Kr00k) vulnerability.
Version 18.104.22.168.385.20433 is now available but I didn't have an opportunity to test it.
This issue took several hours to resolve! Maybe this post will save someone some time.