Thanks for posting and a warm welcome to the community
Sorry to hear about the slow speeds you're getting while connected to a TP Link device. May I ask what device you're referring too, a router or a power line adapter? It's good to know that your direct connection is performing well, if you let us know what device you're connecting via, we will gladly help further.
Catch you again soon,
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there are a couple of possibilities: depending on max powerline speeds these could be a little better than 2/3 of VM router speed (you still get some degredation due to electical lines. i also assume you have power line connected direct to the socket not to an extension of any sort otherwise connection speed could suffer. lastly if all the sockets are on the same circuit breaker circuit it should be good otherwise further deterioration will occur since the powerlines are trying to process through more than one breaker. on connection try click pairing on the powerline first at the router connection then the next nearest till you reach the last. this will effectively clone the router settings password and all. to reset disconnect powerlineand power it down for 30 sec to reset.. good luck troubleshooting.