powerline devices are sensitive to interference and other factors.
Things that can add interference or slow them down:
1. long runs. You might think powerline devices are right next to each other but if their on seperate circuits, its a long run back to the junction box and back to the other powerline device
2. old/bad wiring
3. gfci outlets
4. surge protectors
5. arc fault circuits
6. old/bad electronics
7. poorly made electronics
8. some cell phone chargers
9. high draw appliances
Powerline works amazing on the same circuit. Once you go across circuits, it can drop significantly in speed. I'd take a look at a few reviews to see how they compare. But just keep in mind, your home might have things causing issues versus what the test is showing.
