If your application requires subsecond response, and you're only getting subsecond response some of the time, then you obviously want to know why. Part of that is checking to see if the database and queries are doing their job.
now that log_line_prefix can display the query_id, it could also be used to get a limited version of what log_min_duration_statements does (you can know what query was but not the exact parameters), but in a way cheaper way. of course it assumes that you have something like pg_stat_statements installed and that you need some more granularity than regular snapshots of its information.
