Optimize topic splitting

Splitting topics requires move_posts. Move_posts calls update_post_info before it exits from function_admin.php.

Update_post_info has a query which takes very-very long time to be executed (sometimes more than 30secs). This is it:

// Now, let us collect the user/topic combos for rebuilding the information
$sql = 'SELECT poster_id, topic_id
FROM ' . POSTS_TABLE . '
WHERE ' . $db->sql_in_set('topic_id', $topic_ids) . '
AND poster_id <> ' . ANONYMOUS . '
GROUP BY poster_id, topic_id';
$result = $db->sql_query($sql);

Is there a way to optimize it somehow?

Cheers,
aig