$result = $db->query('SELECT t.id FROM '.$db->prefix.'posts AS p INNER JOIN '.$db->prefix.'topics AS t ON t.id=p.topic_id INNER JOIN '.$db->prefix.'forums AS f ON f.id=t.forum_id LEFT JOIN '.$db->prefix.'forum_perms AS fp ON (fp.forum_id=f.id AND fp.group_id='.$pun_user['g_id'].') WHERE (fp.read_forum IS NULL OR fp.read_forum=1) AND p.id IN('.implode(',', $search_ids).')'.$forum_sql.' GROUP BY t.id', true) or error('Unable to fetch topic list', __FILE__, __LINE__, $db->error());
(...)
$result = $db->query('SELECT p.id FROM '.$db->prefix.'posts AS p INNER JOIN '.$db->prefix.'topics AS t ON t.id=p.topic_id INNER JOIN '.$db->prefix.'forums AS f ON f.id=t.forum_id LEFT JOIN '.$db->prefix.'forum_perms AS fp ON (fp.forum_id=f.id AND fp.group_id='.$pun_user['g_id'].') WHERE (fp.read_forum IS NULL OR fp.read_forum=1) AND p.id IN('.implode(',', $search_ids).')'.$forum_sql, true) or error('Unable to fetch topic list', __FILE__, __LINE__, $db->error());
These two queries around 290-310 in search.php can be quite heavy when the search returns alot of posts.
I suggest adding a feature to the admin to limit the number of results and leaving a message telling the user that not all the results are shown. It's only a matter of chopping off some IDs from $search_ids. Shouldn't be too difficult to implement, but maybe something is needed for the thread view, so you don't end up with one thread containing 30 posts which all contain the search words 
I know there is the "Search all forums" block but personally, I don't like that at all. It would be a pain to search through for example 10 forums. I would just make a new thread asking for the information instead.
