Nintendo knows that there are Wii U consoles that are freezing out there. They also know that some people find the transitions between menus a bit too slow. Here's what Nintendo has to say about both topics.
What to do:
Instances of system freezing are rare and unlikely to occur. If your system has frozen unexpectedly, do the following:
Reset your system by unplugging it from the wall for 10 seconds.
Ensure your system is kept current with available system updates
We are aware that users are experiencing load times when they launch or switch between applications. We are exploring ways to enhance features for consumers’ overall experience.