I just fixed a fairly serious performance problem with postgresql-libpq's binding to PQescapeStringConn; in was exhibiting a non-linear slowdown when more strings are escaped and retained.
I'd like to point out a somewhat related bottleneck in postgresql-simple (but not postgresql-libpq). Every PQescapeStringConn or PQescapeByteaConn call involves a withMVar, which is about 100ns on the threaded RTS on my system. Taking the Connection lock once for the whole buildQuery call might be much faster, especially for multi-row inserts and updates.