I can’t confirm or deny this since I already updated mine, but the following comes from a reddit post by user hlazlo:
“Just a heads up for people who want to play their new console immediately and still update it.
Last night, I set mine up and hit “cancel” at the first mention of the update. We played Nintendoland for a while and then I went to update it. To my surprise, it skipped downloading and simply started installing it. It took less than 15 minutes.”
Others in the comments went on to confirm this, and I can confirm that the Wii U will download other, game-specific updates in the background. If you’re worried about the hour-long update taking a while, you may be able to let out a sigh of relief yet!