Nintendo and its lack of any reasonable sense when it comes to modern technology.

[CityOfNoobs]

You are obviously not getting an optimal wireless connection to the Wii U, there are a ton of setting you could try messing with, or you can just go get an ethernet adapter and be done with the worry.

For what its worth I use the ethernet adapter and downloaded/installed AC3 in 8 hours (12 Mbps connection speed).