@TrickyEarlobe 

Thanks for your information. The Deco M9 plus should be working fine cause the mobile devices get full speed; we will do further analysis and try to fix it. 

1. What is the current network diagram? 

2. With Macbook connected to the Deco M9 plus by wired cable and wireless, please test the speed it can get respectively. In the meantime, please click here to the wired/wireless negotiation speed. 

Note: The steps are the same, while you need click ethernet connection. And please show us the screenshots of the results. 

3. Try to power off the slave Decos and connect the main Deco to the modem only, after that, please test the speed you can get from the Macbook.

4. What is the model number of the 3 Macbook and what about the built-in network adapter? 

Thanks a lot.