> But again, there is no free lunch: copying data is slow. Very slow. Of course you can optimize data representations, avoid copying binary blobs (they are ref-counted in Erlang) because it would be untenable. But it will still be horribly slow.

You don't have to copy the data; it is immutable. Why copy something that isn't going to change? Are we being charged for empty RAM? Different objects can share the same structure. Famously, this is what Clojure does. Theoretically it can be faster than mutating objects because you only have to write the parts that are changing (which is the same as a mutable object), you lose something to overheads (might be nearly negligible) and have enormous gains in situations where you might need a copy of an object for some reason because that is free; there isn't any reason to actually do a copy unless the data itself is going to be mutated.