When releasing a new application (or feature) I've always loaded each table in my development environments database with a few million rows. Tools like Python's Factory Boy [1] or Ruby's Factory Bot [2] help get the data loaded.
After the data is loaded up, start navigating through the application and it will become evident where improvements need to be made. Tools like Django Debug Toolbar [3] help expose where the bad ORM calls are or also by tailing Postgres log files.