Turns out, busy waiting in BEAM is an optimization that ensures maximum responsiveness. In essence, when waiting for a certain event, the virtual machine first enters a CPU-intensive tight loop, where it continuously checks to see if the event in question has occurred.

In our test, we found that BEAM’s busy wait settings do have a significant impact on CPU usage. The highest impact was observed on the instance with the most available CPU capacity. At the same time, we did not observe any meaningful difference in performance between VMs with busy waiting enabled and disabled.

source: L