@doppiak 

First I wanna clarify that the powerline link rate is not the actual speed you can get from the powerline adapters, while it still affects the atcual speed you can get; generally speaking, it is 30-40% of the powerline link rate, except this, it still varies due to the actual network environment. 

Back to your case, we can do some troubleshooting to locate this issue and try to fix it. 

1. Please test the speed you can get from the TL-PA9020KIT with computer connected to it only. You can run the speed test for 3 times. 

2. Please click here to verify the wired negotiation speed of the computer. 

Note：The steps are the same, while you need right click on local area connection or ethernet connection. 

3. Try to move them into the same room to test the speed, or change another wall sockets to plug them.

Below is a guidance you can refer to narrow down the issue. 

https://www.tp-link.com/support/faq/2231/

Best regards.