It is recommended to stick with WP_query for grabbing WordPress posts; but, you can build your own queries directly from the postmeta table, then iterate through the results.
Depending on the query, this may be faster (Not often; WordPress works hard to make their database queries as fast as they can be).
global $wpdb;
$table = $wpdb->prefix . 'the_table_you_want';
$sql = $wpdb->prepare( 'SELECT id FROM ' . $table . ' WHERE some_field = %s AND another_field = %d;', 'some_value', 42 );
$results = $wpdb->get_results( $sql );
foreach( $results as $result ):
//do stuff with $result ( it is an object, not an array )
endforeach;
ALWAYS use prepare when constructing your own queries.