Apple silicon Macs deliver impressive performance largely due to their Efficiency (E) cores, which manage background tasks without interfering with user applications running on the Performance (P) cores. When you start an Apple silicon Mac, Activity Monitor shows a flurry of activity among the E cores as they handle tasks like Spotlight indexing, Time Machine backups, and various system processes. During this time, the P cores remain idle, ready to power user applications without disruption. This architecture contrasts sharply with Intel Macs, where high CPU usage can slow down performance. Apple’s implementation of Performance and Efficiency cores stems from the big.LITTLE architecture, first introduced in the iPhone 7. The allocation of threads to these cores is guided by Quality of Service (QoS) metrics, allowing the system to prioritize foreground tasks on P cores while relegating background processes to E cores. This design prevents background tasks from bogging down performance, enhancing user experience and battery lifespan. Even with thousands of threads running in the background, macOS maintains a clear distinction between foreground and background tasks. The system doesn't promote background threads to P cores, which keeps user applications responsive. Apple's approach encourages developers to create modular applications that can run processes independently, taking advantage of the E cores. The M-series chips have evolved, now featuring multiple E cores to further boost efficiency and performance, ensuring that background operations don’t hinder the speed of active tasks.