Table for actual values
I edited the Query to the following:

EXPLAIN UPDATE jobcontrol.actual_values a
    JOIN jobcontrol.processing p ON a.para_id = p.para_id
    SET a.value = (select p.value from jobcontrol.processing p WHERE a.para_id = p.para_id ORDER BY t_ns DESC LIMIT 1) 



The JOIN ON makes the recordset smaller (DB checks only the parameters which are also in processing, not every single one. The EXPLAIN however indicates that we at least use an index now)
In the SELECT : Replacing the MAX() with a ORDER BY ... DESC LIMIT 1 should boost the performance a bit, especially for big tables. If MAX() uses a filesort (like ORDER BY ... DESC does) nothing should change.

+----+--------------------+-------+--------+---------------+---------+---------+----------------------+------+-----------------------------+ 
| id | select_type        | table | type   | possible_keys | key     | key_len | ref                  | rows | Extra                       |
+---+--------------------+-------+--------+---------------+---------+---------+----------------------+------+-----------------------------+
|  1 | PRIMARY            | p     | index  | PRIMARY       | PRIMARY | 12      | NULL                 |   36 | Using index                 |
|  1 | PRIMARY            | a     | eq_ref | PRIMARY       | PRIMARY | 4       | jobcontrol.p.para_id |    1 | NULL                        |
|  2 | DEPENDENT SUBQUERY | p     | ref    | PRIMARY       | PRIMARY | 4       | jobcontrol.a.para_id |    2 | Using where; Using filesort |
+----+--------------------+-------+--------+---------------+---------+---------+----------------------+------+-----------------------------+