Update and delete executors for Class Table Inheritance (JOINED) are extremely slow on MySQL platform. It is most probably due to use of subselect on the temporary table.
The slowdown is really significant as the table size increases. As an example, lets have a root entity with one subclass:

/**
 * @Entity
 * @InheritanceType("JOINED")
 * @DiscriminatorColumn(name="discr", type="string")
 * @DiscriminatorMap({"root" = "Root", "a" = "SubA"})
 */
class Root
{
	/**
	 * @Column(type="integer")
	 * @Id
	 * @GeneratedValue
	 */
	private $id;

	/**
	 * @Column(type="integer")
	 */
	private $xyz;
}

/**
 * @Entity
 */
class SubA extends Root
{
	/**
	 * @Column(type="integer")
	 */
	private $foo;
}

Now lets perform a simple DQL UPDATE:

UPDATE Entities\Root r SET r.xyz = 123 WHERE r.id > ?

(note: always the upper half of entries)
Which creates following SQLs:

CREATE TEMPORARY TABLE Root_id_tmp (id INT NOT NULL)
INSERT INTO Root_id_tmp (id) SELECT t0.id FROM Root t0 LEFT JOIN SubA s0_ ON t0.id = s0_.id WHERE t0.id > 25000
UPDATE Root SET xyz = 123 WHERE (id) IN (SELECT id FROM Root_id_tmp)
DROP TEMPORARY TABLE Root_id_tmp

The time spent on this on MySQL 5.5.17 and PostgreSQL 9.1 is:

As you can see, MySQL is drastically slower on even relatively small tables. This currently makes Doctrine unusable for this type of inheritance on MySQL. The solution probably would be to avoid subselect in WHERE clause in Doctrine\ORM\Query\Exec\MultiTableUpdateExecutor and Doctrine\ORM\Query\Exec\MultiTableDeleteExecutor.

Feel free to try/modify the test script yourself, it's here.