@Refractory,

From the network settings on Windows, or from the Network Adapters option from the control panel. You should be able to see the link rate/speed of the connection in the properties or advanced settings for the Wi-Fi adapter. If the numbers listed are the same between the two devices.

I would also recommend updating or even reinstalling the drivers for your network card from the manufacturer's website, updates to network cards come relatively often and usually have a large amount of compatibility fixes.

You may also take a look at the QoS settings for your network to see if they are set below what you expect, or if the laptop had a set priority for speed.

After you connect the cable, have you tried restarting the PC to see if the connection will be better after it renegotiates with the network?

Laslty, on the Windows PC - My go-to reset for odd connection behaviors is to go to the command prompt and use the commands 

"ipconfig /release"

then wait

"ipconfig /renew"

this will disconnect your PC from the network and force the router to give the PC a new IP address. This often has the side effect of clearing any properties that may have been remaining from the previous connection.