Install the extension pg_stat_statements with the SQL command
CREATE EXTENSION pg_stat_statements
You may want to make sure you create this by using an appropriate user (such as the user your application uses or some dba account). Be aware that whichever user creates the extension will also own it.
This will require a server restart for it to be usable (because you have to change the config in postgresql.conf a tiny bit that affects memory consumption), but it will give you statistics on all queries.
Please see the documentation page for more information about what pg_stat_statements provides. See here for a quick reference on configuration of the postgresql.conf config file for pg_stat_statements.
Once you have it installed and running, you can view statistics by querying the pg_stat_statements view..
select * from pg_stat_statements;
select * from pg_stat_statements where total_time / calls > 200; -- etc ..