He kinda did. He writes that they changed from Erlang to C because of problems in Erlang. He wanted some safety or tools to help him find problems easier (actually problems in the language or libraries they used). They might develop slightly faster now in C than they did before in Erlang but I doubt it and the rewrite took time that probably cost them 2 months of inproductivity compared to not rewriting it. As for running speed they of course want that. Can't remember if they actually thought that low speed was problem with the earlier Couchbase.