Your powerline speeds will also be affected by they way your electrical system is set up in your home.
You will get the highest speeds from powerline if the sender and receiver device are plugged into the same circuit and the same consumer unit/fusebox. Different circuits (say across floors of the house) can significantly drop speeds.
On a ring main circuit you will get a drop in speed if the socket you are using is a spur from the ring.
Also the speed can be affected significantly if either the sender or receiver is plugged into an extension cable or extension/slitter (e.g. 4-way power splitter block). The sender and receiver should be plugged directly into a wall socket.
I have used powerline in the past (with a nominal max 500 Mbps speed) and the very best I got from that with 90 Mbps under optimum conditions.
If you want to test your powerlines and network cables, see if you can rig up a test arrangement where you plug in the powerlines side by side into a double socket and see what speeds you get that way. If you still get 10 Mbps then you may well have a powerline/cable problem. If you get a greater speed it may be the electrical wiring which is causing your speed drop.
I eventually wired up some permanent network cables to get full speeds everywhere in my home.
