What could diminish MySqls performance?

Question

Basically we are very happy with Mysqls performance, similar queries are completed in a split second. Now we face a problem with this query

SELECT dc.id,dmr.art_id 
FROM dmr 
JOIN dma ON dma.id = dmr.dml_id
JOIN dc  ON  dc.id = dma.dc_id
WHERE  dmr.art_id = 2285

It takes 50 seconds to fetch 5021 rows. A missing index is probably the most common cause for issues like that. So I preceded the query by EXPLAIN and got this query plan which shows that only indexes are used no sequential scans.

The tables dmr and dma have 3 million rows each, dc has 6000 rows.

+----+-------------+-------+--------+-------------------------------+----------------+---------+--------------------+------+-------------+
| id | select_type | table | type   | possible_keys                 | key            | key_len | ref                | rows | Extra       |
+----+-------------+-------+--------+-------------------------------+----------------+---------+--------------------+------+-------------+
|  1 | SIMPLE      | dmr   | ref    | FKC33D5199F17E1825,ix_art_ref | ix_art_ref     | 5       | const              | 5021 | Using where |
|  1 | SIMPLE      | dma   | eq_ref | PRIMARY,FK8C6E1445153BBDC9    | PRIMARY        | 8       | dev.dmr.dml_id     |    1 |             |
|  1 | SIMPLE      | dc    | eq_ref | PRIMARY                       | PRIMARY        | 8       | dev.dma.dc_id      |    1 | Using index |
+----+-------------+-------+--------+-------------------------------+----------------+---------+--------------------+------+-------------+

What could cause this problems?

MySql Version is 5.5 using InnoDB as engine. (Only default parameters on windows).

EDIT

When I remove the where clause, mysql returns the (huge) result set immediately. The query plan in this case looks like:

+----+-------------+-------+-------+----------------------------+--------------------+---------+------------+------+--------------------------+
| id | select_type | table | type  | possible_keys              | key                | key_len | ref        | rows | Extra                    |
+----+-------------+-------+-------+----------------------------+--------------------+---------+------------+------+--------------------------+
|  1 | SIMPLE      | dc    | index | PRIMARY                    | FKAEB144C64FA71464 | 9       | NULL       | 4037 | Using index              |
|  1 | SIMPLE      | dma   | ref   | PRIMARY,FK8C6E1445153BBDC9 | FK8C6E1445153BBDC9 | 9       | dev.dc.id  |  263 | Using where; Using index |
|  1 | SIMPLE      | dmr   | ref   | FKC33D5199F17E1825         | FKC33D5199F17E1825 | 9       | dev.dma.id |    1 | Using where              |
+----+-------------+-------+-------+----------------------------+--------------------+---------+------------+------+--------------------------+

Answer 1

Maybe the tables are very fragmented and MySQL has to go all over the disk to fetch those 5021 rows?