Speed is an issue of the full time supervisor as his bonus relies on certain metrics. It's an issue for the part time supervisors because they don't have union protection for their jobs when the workers aren't fast enough. Aside from intentionally working too slow, which could be considered stealing time, union workers have no speed requirements.
What most part time workers don't realize is: they will get faster the longer they do a job, shortcuts taken at the beginning of the shift will likely cost time over the course of the shift. For instance the biggest slowdown I see people cause for themselves is stacking out packages. Sure it's faster than loading them, but then they spend extra time squeezing by them slowly to keep from knocking the stack over. Not only that, but you still are going to have to spend the time to load them later.
Just follow the methods, work safe, and you'll get faster.