Postgres de/serialization appears to be 2x slower than postgresql-simple

[LaurentRDC]

See discussion here: #767

[LaurentRDC]

There's a repository to reproduce the benchmark here. First step would be to run this benchmark with profiling turned on

[LaurentRDC]

I was able to run a profiled build of the repository attached above, and got the following profiling (unzip and read in https://www.speedscope.app/)

beam-demo.zip

This shows that in addition to the expected Database.PostgreSQL.Simple.fromField, there's quite a bit of extra time spent in Database.Beam.Postgres.Connection.runPgRowReader. (specifically at Database/Beam/Postgres/Connection.hs:134:3-44)

[LaurentRDC]

It looks like the performance problem is entirely contained within this function.
I'm not familiar enough with this part of the codebase to suggest a fix yet. I'll have to try out some things and profile as I go