Why is my powerline ethernet adapter slower than advertised?

The advertised speed  is the theoretical physical rate. Actual throughput will vary. Network conditions and environmental factors, including volume of network traffic, and network overhead affects actual throughput rate.

Specific factors which may reduce your effective speeds are power fluctuations EM interference from induction motors such as refrigerators, washer/dryer, microwaves, hair dryers, etc.and poor grounding.
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