Is the extender using wireless to extend or a wire? If it is using wireless then you should be able to plug a cable into the Ethernet socket on to the tester as that would show what speed the connection between the plugs is capable of (it could be that it is the wireless link between them that is slowing it down).
If you connect by wireless close to the wired airport express do you get higher speeds?
Also - if you turn off the airport expresses and just connect to the 8800NL via wireless what speed do you get?
Edited by ian72 (Thu 31-Mar-16 10:26:27)