Time Capsule slow on Ethernet
If you connected the Ethernet cable while the backup was running, it is probably still utilizing the wireless interface. It can not re-route an established connection in the middle of the backup.

Start by cancelling the backup. Then do one of the following:


Disable the Wi-Fi interface on the computer; OR...
Open Network in System Preferences; click on the cog button below the list of network interfaces; click on "Set Service Order"; drag the services so that Ethernet is on top of the list; click OK and Apply.


Now, restart the backup.