You can use the log_statement config setting to get the list of all the queries to a server
https://www.postgresql.org/docs/current/static/runtime-config-logging.html#guc-log-statement
Just set that, and the logging file path and you’ll have the list. You can also configure it to only log long running queries.
You can then take those queries and run EXPLAIN on them to find out what’s going on with them.
https://www.postgresql.org/docs/9.2/static/using-explain.html