Improved slow tasks for MEX

[micheles]

Weeks ago @kejohnso run a calculation for MEX on cole taking over 10 hours (i.e. 5x longer than the right amount of time) due to a slow task. This is now finally fixed. Here are some numbers on the machine engine192:

# before vs after
| operation-duration | counts | mean     | stddev | min      | max      | slowfac |
|--------------------+--------+----------+--------+----------+----------+---------|
| classical          | 416    | 907.9    | 87%    | 234.4    | 11_394   | 12.5502 |
| classical          | 377    | 998.3    | 67%    | 87.0405  | 2_442    | 2.4465  |

The slowest task is 4.7x faster now.