Disable (or limit) queries when certain content (or data) is not needed (or showed)

You can filter 'posts_fields', check if it is the main query and limit the queried fields to … whatever you need.
See WP_Query::get_posts() in /wp-includes/query.php for details and side effects.

For debugging the queries I recommend the plugin Debug Bar. If you add …

define( 'WP_DEBUG',         TRUE );
define( 'SAVEQUERIES',      TRUE );

… to your wp-config.php you can see the resulting query and how it affects later queries.