Be sure the powerline is on the same side or phase of the AC circuits. This is well discussed, on many sites so I'll pause here.
I have recently been lucky enough to get super-fast fibre optic in my area, with a maximum download speed of 160Mbps (Upgraded from 20Mbps).
My downstairs computer (which uses Ethernet cable) Is capable of sometimes upwards of 160Mbps download. (The router is right next to this computer).
On my xbox one which is on the third floor of my house. When I run a 30m Ethernet cable from my xbox one to my router I test the network statistics of the xbox one. I am able to get 160 Mbps as I expected.
But the problem is I don't want a wire running all the way down the stairs so I have used a Powerline (500Mbps). When I test the network details on the xbox one, the download speed fluctuates between 25Mbps - 35Mbps. Compared to the Ethernet cable which produces a stable 160Mbps download this seems very slow.