Michelle Konzack writes:

Well there's your problem right there. PAM does this in a rather simplistic, primitive way. Each time someone wants to look up a userid, it logs in to Postgres, runs the query, and logs off. Lather, rinse, repeat. Every time. There is no persistent component to libpam-pgsql. The PAM library reads the PAM configuration file, loads the specified PAM modules, which are shared libraries, invokes the requested functions, and unloads the module.

You need to switch to Courier's native PostgreSQL driver, which will log in once, and keep a persistent database connection open to handle all authentication requests that come in. You can control how many database connections are open, in parallel, by controlling the number of authdaemon processes, to scale with your load.

This completely eliminates the need to log in and log out for every authentication request, which will make a big difference.