M1: QoS of macOS task is what makes it snap
Apple’s M1 is a fast CPU—but M1 Macs feel even faster due to QoS

          
          
Unfortunately, while cfq did in fact measurably improve maximum throughput, it did so at the increase of task latency—which meant that a moderately loaded system felt sluggish and unresponsive to its users, leading to a large groundswell of complaints.
Although cfq could be tuned for lower latency, most unhappy users just replaced it entirely with a competing scheduler like noop or deadline instead—and despite the lower maximum throughput, the decreased individual latency made desktop/interactive users happier with how fast their machines felt.
After discovering how suboptimal maximized throughput at the expense of latency was, most Linux distributions moved away from cfq just as many of their users had. Red Hat ditched cfq for deadline in 2013, as did RHEL 7—and Ubuntu followed suit shortly thereafter in its 2014 Trusty Tahr (14.04) release. As of 2019, Ubuntu has deprecated cfq entirely.
QoS with Big Sur and the Apple M1
When Oakley noticed how frequently Mac users praised M1 Macs for feeling incredibly fast—despite performance measurements that don't always back those feelings up—he took a closer look at macOS native task scheduling.
MacOS offers four directly specified levels of task prioritization—from low to high, they are background, utility, userInitiated, and userInteractive. There's also a fifth level (the default, when no QoS level is manually specified) which allows macOS to decide for itself how important a task is.
These five QoS levels are the same whether your Mac is Intel-powered or Apple Silicon-powered—but how the QoS is imposed changes. On an eight-core Intel Xeon W CPU, if the system is idle, macOS will schedule any task across all eight cores, regardless of QoS settings. But on an M1, even if the system is entirely idle, background priority tasks run exclusively on the M1's four efficiency/low-power Icestorm cores, leaving the four higher-performance Firestorm cores idle.

          
                  