“Ordinarily, I/O operations on one disk device don’t throttle those on another device.”
Copy one external disk to another with dd and your whole Mac grinds to a halt, Terminal included. The kernel files every block device under /dev in the same throttling bucket as the main disk. That fallback value was changed in Lion, back in 2011. Jacobson found it with spindump and dtrace, and the fix is two constants. Apple shipped the bug for a decade.