So I was getting woeful to negligible wi-fi connection speeds from three different routers: my ISP-provided modem router; my Apple Air
port Extreme and my cheap mesh network when trying the MacBook Air in the bedroom. Checked the speed of the Ethernet-connected M4 Mac mini: it was fine. This is connected to an Ethernet switch which in turn is connected to the ISP modem router.
A simple fix. Turn the modem router off and back on again, waith 10-15 minutes for connection to be reestablished and try again. Bingo, all wi-fi networks were back up to their normal speeds.