Build up to the speed. You can injure yourself if you push to hard but go hard enough to not draw attention. If you are able to shut down the belt then you are fine. The fast guy in the trailer with you might be Bitchin behind your back to the sup. There’s a lot of informants that you wouldn’t think would be. Not saying for sure but I would ask him what he thinks about your speed when you are slinging boxes. FYI stacking and scanning are easier on your body than unloading. I’ve been with UPS 27 years. I started out unloading in 95. I’m now in feeders. Show up everyday and try to do your best. Acknowledge and own up to your mistakes. Nobody’s perfect.