Interesting... just ran a speedtest.net test, and did get my full download speed (35Mbps) on battery, despite showing a 6Mbps connection that did ramp up a bit to 58Mbps during the download portion of test. The upload portion however is still much slower on battery... plugged in I'm right at 5Mbps, what I always get on all devices; upload speed however is only 1.1Mbps on the SP3 when on battery.
This is using whatever drivers were installed during Windows Update and using an updated Netgear R7000 5.0ghz AC network.