Optimizing ORDER BY for simple MySQL query

Question

I'm trying to optimize this really simple query, and it gives me grief for a day now :(

Seems pretty straightforward, I need to select with a JOIN from 2 tables, and get top X results sorted in a particular order. Here's the query:

SELECT * FROM `po` 
INNER JOIN po_suppliers s ON po.supplier_id = s.id
ORDER BY po.id ASC
LIMIT 10

However, it runs really slow (half a second to 2 seconds). Here are table structures:

CREATE TABLE `po` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `supplier_id` int(11) NOT NULL,
  PRIMARY KEY (`id`),
  KEY `supplier_id` (`supplier_id`)
) ENGINE=InnoDB AUTO_INCREMENT=457790 DEFAULT CHARSET=latin1

CREATE TABLE `po_suppliers` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=9386 DEFAULT CHARSET=latin1

Running EXPLAIN reveals the issue:

+----+-------------+-------+-------+---------------+-------------+---------+--------------+------+----------------------------------------------+
| id | select_type | table | type  | possible_keys | key         | key_len | ref          | rows | Extra                                        |
+----+-------------+-------+-------+---------------+-------------+---------+--------------+------+----------------------------------------------+
|  1 | SIMPLE      | s     | index | PRIMARY       | PRIMARY     | 4       | NULL         |  480 | Using index; Using temporary; Using filesort |
|  1 | SIMPLE      | po    | ref   | supplier_id   | supplier_id | 4       | sergiis.s.id |    1 | Using index                                  |
+----+-------------+-------+-------+---------------+-------------+---------+--------------+------+----------------------------------------------+

Can you help me figure out how I can optimize this to run faster? I have index on the column I sort by. I have index on columns I join by. If I remove the JOIN - it's crazy fast. If I remove ORDER BY, it's crazy fast. Why am I getting this dreaded temporary + filesort?