The difference is real world compared to lab. In theory if you use a 802.11ac you would be slowed down by usb 2.0 speeds. When it has been actually tested it seems it is almost impossible in real world situations to get 802.11ac to run faster than maximum speed of usb 2 even when you use the 802.11ac adapter on a usb 3 connections.
And of course this assumes you are transferring to something inside your house since nobody has internet connection that fast