Windows – How to fix Ethernet speed being stuck at 10 Mbit in Win10

You need to check the cables

• There are cables and cables. Since you have two, switch them between the machines and see what happens
• Sometimes there are problems with the cables not being plugged in properly (esp. if the "hook" doesn't work or is missing). While I'd expect no connection, check this anyway.

That's not wireless... those adapters are using your electrical wiring as a medium to transfer the data.

This technology is known for being less than reliable, especially if the two ends of the connection are on different circuits. More than that, a lot of power lines have a lot of noise in the wire that can significantly reduce that actual vs theoretical data throughput.

Now let's look at some other limiting factors. If either computer only has a 100Mbps card, that is as fast as you'll go for the transfer. We also need to mention the uppercase B in MBps vs the lowercase b in Mbps. Whenever you see a lowercase b, you need to divide the number by 8 to get a better idea of how many bytes (vs bits) you can transfer.

So a 100Mbps link from the computer could at most supply your powerline adapter at 12.5MBps, and a certain amount of additional loss is normal for noise, collisions, and TCP overhead. The speed at which your source hard drive is able to read the data may be a factor as well, especially if you're moving data from an older computer to a newer one (which a 100Mbps adapter would suggest). One more potential limitation is the duplex setting of the connection. Most wired connections use two pairs of wires to allow both sides to transmit and receive at the same time. It's possible and even likely that your powerline adapter is only half-duplex, and cannot send and receive at the same time. This will result in more packet collisions that will further slow the transfer. Now factor in high noise in your powerlines, and 3.5MBps seems downright reasonable.