As a Repeater (not Cable) over 5Ghz for both WWAN and WLAN, my GL-AR750S-Ext only gets 40-50Mps. A single 5GHz radio cannot transmit and receive both ways at the same time, so speed is typically cut at least in half.
In my case, my WLAN often drops before speedtest.net finishes.
I do not work for and I do not have formal association with GL.iNet