The use of the background queue is a historical artifact from when Santa had its own kernel extension with separate in-kernel queues for processing AUTH & NOTIFY type events. With the move to ES and the larger number of event types that we now notify on, running at the background QoS carries a small risk that the thread processing these events is not given a chance to run often enough that the queue grows and increases memory usage.
The use of the background queue is a historical artifact from when Santa had its own kernel extension with separate in-kernel queues for processing AUTH & NOTIFY type events. With the move to ES and the larger number of event types that we now notify on, running at the background QoS carries a small risk that the thread processing these events is not given a chance to run often enough that the queue grows and increases memory usage.