PG 9.5 will improve this situation somewhat, e.g., builtin functions like jsonb_set(), and overloading the '-' operator for jsonb values:

http://www.postgresql.org/docs/devel/static/functions-json.h...

That said, if you're doing a lot of mutation of large JSON documents stored in a single Postgres row value, the storage/concurrency control behavior still won't be ideal.

Actually without providing some benchmark results, the statement, that PostgreSQL has performance problems because of on-copy updating of JSONB, is groundless.