Without re-using prepared statements or portals, extended protocol is always slow because it requires more messages exchanges than simple protocol. In pgbench case, it always sends parse, bind, describe, execute and sync message in each transaction even if each transaction involves identical statement ("SELECT 1" in your case).
See the manual for the protocol details.
I'm well aware of how the extended protocol works, but it seems odd for a 30% increase in processing time to be the result exclusively of processing 5 messages instead of just 1 - it doesn't seem like that big a deal (although I may be mistaken). I was imagining that there's something more fundamental in how the protocol or PostgreSQL state is managed internally, that would be responsible for the slowdown.
