It's all green threads, so you don't have real context switches and the messages don't have to be passed between (real) processes. Of course there are downsides to that kind of approach: good luck calling a C library function that you have to pass a callback into, anything else where you need precise control over your OS-level threading is difficult (and while BEAM has a good reputation, there's always the risk of e.g. priority inversions happening), and if the VM scheduler doesn't handle your workload well then you're limited to whatever control they give you.
