This is just my own opinion as how you should use UWorld.
UWorld is first and foremost a learning tool. If you want to practice under timed conditions, take an NBME. I feel like you should learn how build a solid foundation before working on speed and test acumen. Doing it system by system allows you to compare and contrast the ways questions can be asked for similar diseases as well as practice what you recently learned. That being said, everyone learns differently and there is no single best way to use UWorld. Only you know which way you learn best and how you should use UWorld.
With 2nd year curriculum → tutor, system based.
Dedicated period without a solid foundation → tutor, system based.
Dedicated studying period with a solid foundation → timed, random blocks.