MacBook Air , like many devices have cases that are far too slim to have an Ethernet port , so you can’t plug the device directly into the router LAN socket directly , it requires an adapter often USB-C to Ethernet, so the USB plug can fit into a port on the device case, and the cable from the router connects to Ethernet socket side of the adapter, presumably you are using such an adapter if you are measuring speed with a wired connection , was it supplied by Apple ? , or some ‘Chinese’ Amazon special ? , perhaps you need to source another one just to eliminate that as the potential bottleneck .
