Firstly, the numbers for
nice work the other way round, i.e. 20 means run at the lowest priority. Secondly, this can do no good -- programs don't slow down to be awkward so that a
nice priority can persuade them to hurry up or something -- there is always some other reason for it.
In this case the process is spending too much time waiting for database interactions to complete and change of priority cannot change that behaviour. But if it hadn't been that it would have been some other I/O activity.
Finally, on unix, 20 or more is equivalent to 19, which is the maximum-valued i.e. lowest priority.