If you want massive performance, you most likely want to avoid erlang; if you want massive scalability, erlang might very well be a great fit. I think it's quite different.
Performance in terms of request throughput you can serve per minute for some given set of CPU and RAM.
Erlang processing is slower than C by quite a bit but the reduced threading costs tends to make up for it when you are in a scenario where you need to serve multiple requests simultaneously.
On an apples to apples comparison well written C is going to beat erlang on this every time but once you start getting into threading, mutex locks etc. the equation starts to shift more towards Erlang favor _as long_ as each request you are serving is not highly computational in nature and would involve semaphores, and so on for thread management in c.
