Erlang's core reason for existence is to control telephone switches, which had two independent general purpose computers connected to the physical switch. So reliability, redundancy, recovery, and fault isolation were the core needs; that drove the design for isolated processes with message passing between them. Because Erlang was in the control plane, and only managing the signal path, not passing the signals itself, there wasn't a big need for speed, as long as it wasn't too slow

Fast forward several years, and isolated processes turns out to be a great fit for large SMP systems, and Erlang/beam is now doing signal path work in a lot of places. Erlang tends not to put explicit limits, but some techniques are going to fail at large scale; ex: if you have 50,000 processes across many nodes, sending the same message to each of those processes is going to be slow; sending one message to each node and fanning out from there is going to be faster; in no small part because you've reduced the network bandwidth you're using.

The nice thing when you hit Erlang scaling limits is that almost everything you need to fix is going to be in a pretty simple state. You're not going to find many things that are layers of optimizations on top of hacks on top of optimizations --- they do a good job of keeping things simple, and not optimizing until it's needed (and even then, they usually pick simple optimizations). Keeping things simple goes a really long way (especially with today's enormous servers).

Edited to add: I don't think they've even needed to tweak the vm yet either, just their user space code. That's pretty huge too.