[_ck_]

Not extensively tested but this will do it without a core hack

(once ALTER TABLE bb_topics ADD INDEX (topic_time) is done)

<?php

/*

Plugin Name: Topic Time Index

*/

add_filter('get_latest_topics_join','topic_time_index',99999);

function topic_time_index($join) {return " $join USE INDEX(topic_time) ";}

?>