Slow Intenet speed using Powerline

Power lines are notoriously noisy. The noise comes from multiple sources including those within the hime and those outside the home. Think about the trip your power makes to reach any given socket. It starts at the generating system, goes through miles of power transmission lines, through a transformer, and to every socket, light, and appliance in the house. While you're only transmitting between two sockets, all of the power lines can pick up noise that noise will be present in the entire system.

All of those power lines can also act as antennas to pick up noise and anything connected to your house wiring can be an additional source of noise on those lines. What complicates the issue is that those lines were not designed for signal transmission so they have none of the features of an ethernet cable like outer cable shielding and twisted pairs of wires that act to reduce noise and protect against picking up unwanted signals.

Extracting a data signal from a power line has been likened to trying to hear a dropped pin hit the floor while a symphonic orchestra is playing in the same room.

Another problem for power line signals is that the data must be nearly perfectly received to be valid. Unlike audio or video signals where one thousandth of a second signal error is acceptable because it can't really be perceived, the entire data stream must be correct. To accomplish that the transmitters and receivers use multiple layers of error detection and correction and when an transmission error does occur the data packet must be resent. All of those steps require time and add delays. If the line is particularly noisy the transmitters will slow down the transmission to try to get around the problem.

The gist of all of that is that you can't expect the same speeds from power line data transmission that you will get from an ethernet transmission.

There are a few things you can do to improve the system but not a lot. In many cases it is cheaper to just run an ethernet cable. If that is not possible search "power line noise filter". Your powerline adapters will have noise filters already so the point here is to add filters to other equipment in your home that might be contributing to the noise on the power lines.