Sorry, but I've used all the competition. Wii U is incredibly slow in comparison. Just loading settings takes 5-10 seconds. It's instantaneous on every other console. Ditto for the eshop.
Settings is indeeed the slowest part of the Wii U UI. It used to take 30 seconds to go there. But practically except for the first set up, since launch, I never went to settings except for the times I wanted to change the country (EU here) for the eshop.
As for eshop itself, it's a webpage, it loads as fast as a webpage, sometimes instant sometimes takes 5 seconds, depending on the network status (or if you already download something).
It is not slow, it is not fast, it's functional.