Let say that I have a table with 20 non-null text columns (32 characters long), 20 non-null integers and a blob (bytea, nullable). When inserting records, what is the overhead of setting the blob? Answer: about no difference up to 80ko. From that it get slower.
Testing details: 2000 records (on 2000 transactions), inserted on 16 threads, each of them having a persistent connection. The DB server is on the same machine (Ubuntu amd64), no particular configuration.