The system prioritizes tasks based on their impact on the user’s experience, giving higher priority to work that directly affects the user, such as UI updates. Assigning Quality of Service (QoS) classes to your app’s tasks helps the system schedule work effectively, balancing responsiveness and energy efficiency. QoS classes categorize tasks into four primary levels: User-interactive, User-initiated, Utility, and Background. Developers can apply QoS to operations, queues, dispatch queues, blocks, tasks, threads, and even CloudKit classes. Debugging tools like Xcode and powermetrics can help ensure that the assigned QoS classes are applied correctly. Balancing performance and energy efficiency is key to creating a responsive app.
https://developer.apple.com/library/archive/documentation/Performance/Conceptual/power_efficiency_guidelines_osx/PrioritizeWorkAtTheTaskLevel.html