The speed-up of having not just ethernet connected (to the network extended via powerlines) but ethernet + WiFi (to the same network) is quite pronounced.
(Running latest Fedora Silverblue without any special network tweaking.)
@ainmosni That probably had more to do with the server I was connecting to. Here’s a speedtest I just ran to a server in Dublin.
(But yes, powerline does cut the bandwidth down considerably. Can easily halve it. I can get close to 1Gbit if I connect directly to the main router downstairs.)