RMBP slow download speed

The problem is with the Retina Mac. Are you using Bluetooth also? If you are there is a incompatibility between the WiFi and BT when both are turned on and maybe in use (could be if BT is on but not used the incompatibility doesn't happen).

Both the WiFi chip and BT chip are on the same Daughter card and maybe the antennas for both arre run along side one another (Not sure about the antennas as I have never taken a retina model apart, Yet).

If you do have BT on, whether you are using it for a mouse or keyboard or whatever, turn it off and I bet your dowload speeds increase.

Also when stating speeds Mb (small b) stands for Mega bits and MB (Capitol B) stands for Mega BYTES. There are 8 bits to each BYTE so your ISP speed of 20Mbs (Mega bits a second) is equal to the Win PC downloading at 1-2MBs (Mega BYTES a second) 20Mbs / 8 = 2.5MBs. Which with overhead and internet traffic you get somewhere between that 1-2MBs you are seeing on the Win PC.

Then if this is a NEW MBPr take it back an get another. Apple has a 14 day No Questions Asked return policy. Take advantage of it to get one that works correctly.

Did you reboot the system after turning off the BT? If not give that a try.

Actually there is/was a very long thread on the extremely poor WiFi connections on the MBPr and most were solved by turning off the BT. But if you need BT for an external mouse or whatever then that doesn't really work for you.