Are you power leveling? If so, I highly suggest to simply use the cheat mod and hit level 99 instantly. The grind to level 99 is extremely time consuming, repetitive, mind numbing and not fun at all. Although if you want the prisoners to be faster in general then we might need someone to edit the -1 stamina lock of prisoner so that their stamina can go into the positive and gain more speed. In older version of the game, there used to be a time when healing prisoners or rescuing them before they were touched was the way to rapidly save them since their stamina wasn't locked to -1 and less.