select from table "user" query slow

[entertain]

I wanted to optimize some queries in iTrader and found a problem when loading the latest traders in combination with a huge user table.

A query looks like that:

		$itrader_shame_get = $vbulletin->db->query_read_slave("
			SELECT itrader_total, itrader_pcnt, username, userid, joindate
			FROM " . TABLE_PREFIX . "user
			WHERE itrader_total < 0
			ORDER BY itrader_total ASC
			LIMIT $mainpagelimit
		");

When you have a big user table, the query needs a few seconds to finish.

Do you have any idea how this could be enhanced?