@samwardill 

Thanks for your reply, we will do some troubleshooting to locate the issue.

1. What is the current network diagram? How many Deco P9 do you have and how do you connect them together? 

2. With the 2 slave Decos unplugged, what is the speed you can get from the main Deco by wired cable or wireless respectively? 

Meanwhile, please click here to get the wireless negotiation speed. 

3. Try to connect your computer to the slave Deco units by ethernet cable to verify the wired connection.

4. Try to swap the location of each Deco and rule out whether it is the Deco itself or the location/network environment. Or you can switch the slave Deco to the main Deco and then check the speed you can get from the " new " slave Deco. https://www.tp-link.com/support/faq/2400/

5. Besides, if possible, it is recommended to connect the Deco units by wired cable to give it a shot.

Good day.