Nimnim
The Nim
Actually this is my 5th week. My supervisors say they like me, but just push me to work faster.
So you've not made seniority yet, yes they can let you go because you're in the probation period without any recourse from you. If you keep showing up everyday and put in a solid effort and show even a little improvement you're likely to be ok.
It's been a few years since I was in the unload but depending on the size of the trailer without irregs, it can take between 45 minutes to an hour 15 minutes while still preforming adequately. To their standards, you're going slow, but unless they don't have a demand for more bodies in the area I wouldn't worry as long as you're putting in an honest effort and they can see you're likely to improve.
If you're 5 weeks in though and they look at how you're doing and it's more or less a straight line(or dropping) on their chart of how your production is doing they may end up passing you up and hoping the next person will do better.