@Mox27 Hi there, 

TL-PA7027P and TL-PA7017P are compatible with each other. To isolate the issue, please test 1 Powerline KIT + 1*  extra powerline extender. 

It is recommended to run speed test multiple times on extender PA7020P, speed normally varies in different tests. 

Please do not connect any wired client to the new PA7010P, do you always find PA7020P speed reduce one new adapter is plugged in?

Try a different wall socket for PA7010P which is a bit further from PA7020P and see if speed gets better. 

If still the same problem, may I know your network topology , for example: Router ---PA7020P (main adapter)................. PA7020P ( extender in Bedroom)

Please also share speedtest result when PA7010P is plugged in ( or unplugged) .

Please download tpPLC Utility and see how poweline rate changes when you plug in an extra powerline extender? 

https://www.tp-link.com/support/download/tl-pa7010p-kit/#Utility