Editing wp-includes/feed.php

First of all, never edit a core file. As soon as WordPress is updated, you will lose your changes.

The part you’ve edited already has a mechanism in place to make the change you want without editing that file. All you need to do is add a filter to wp_feed_cache_transient_lifetime to set the lifetime in seconds:

function return_1800( $seconds ){
    // change the default feed cache recreation period to 30 minutes
    return 1800;
}
add_filter( 'wp_feed_cache_transient_lifetime', 'return_1800' );

Place that in your theme’s functions.php file, or within a simple plugin.