That is because the Wii U has to load the HOME Menu to memory and store the game data elsewhere. When you exit the HOME Menu it loads the game data back to memory which causes the small pause. BOTW is more resource intensive than most other games on the system.

Technical limitations

im pretty sure the wii u only has 2-4 gb of ram (2000s laptop equivalent) and while gaming only half is used incase u pull up browser and such