If string processing is a bottleneck in your system, either your system isn't doing anything else interesting to take up CPU time, or you've done something very, very wrong. Serialization is a damn-near solved problem.
In particular, redundant string concatenation and flattening tends to be CPU hog, but IO-Lists automatically flatten all string types during transmission and have already been thoroughly optimized.
Of course, if you ignore the serialization solutions Erlang provides, there is a performance hit.