@Kirainhere 

Hi, does the packet loss happen only when the CPU usage reaches 100%?

On the wireless client devices, please ping the router LAN IP address and also google DNS at the same time to see if there is packet loss when pinging the router LAN IP address or not.

For comparison, you could also run the same tests on a wired computer.

If the issue happens with wireless connections only, please try to change the WiFi channel and channel width to see if it can be improved.

By the way, it is recommended to disconnect the Archer C54 when testing in case the client device connected to the repeater.

  @Kirainhere 

Hi, do you have any UPnP device or application that might need port forwarding and support UPNP? such as multiplayer gaming etc.?

You could try disabling it on Advanced > NAT Forwarding > UPnP page to see if it helps with the packet loss issue.

  @Kirainhere 

Hi, could you please attach the complete log file after the issue happens? we would like to check if there are any WAN disconnections at similar times.

Usually, when WAN disconnects and reconnects, the router will reobtain WAN IP address, and related processes will be restarted including the UPnP.

If you have any WAN disconnecting issue, you could also check LED status of the router, test if your ISP modem is working stable or not, for more troubleshooting, please refer to this thread: What if TP-Link Router Disconnects from Internet?