I nearly quit because of this exact situation with my sup.
Either make loud grunting noises until he leaves your work area or go do what you gotta do to make it stop. I made it stop and since then it's been a much less stressful environment, sup will still sometimes get on me for moving faster but it's no longer all day every day when I'm moving 1300+ packages at 300 PPH. Some sups will always think it's their job to tell you to move faster, but follow the methods, work safe, and they can't touch you. Personally, I like seeing how I can improve my PPH, makes the job "fun" when I can compete with myself, but that's just how I deal with it.
The issue I had with my sup was remarkably similar to what you're describing here. When I started all my coworkers warned me that he was a pain in the ass and to not take him too seriously. I didn't talk back and would just nod and keep working. Nothing changed until I made it change. That's something you'll have to learn here, you'll keep getting stepped on until you bite back, then they'll know when to ease off you. Be like a rattle snake. Make noise and they'll leave you alone.
It got to a point with my sup where on a day when they gave me an extra truck (bulk to clothing store with 400 some pieces, brick loaded through the whole thing), he told me I wasn't working hard enough and that I needed to move faster. This was in the middle of the summer, I was drenched in sweat, and moving my fastest. I put the box I was holding down, told him that I was doing my best, and that his continued harassment was not helping. His response was that he didn't believe me. At that point I just told him that if he didn't believe me then there was nothing I could do to change his mind. I kept working, he left for a bit then came back and we went through the same thing again...so I gave him the spark notes (told him to
off).
It kept going until one day that last straw fell and I walked up to HR to turn in my ID. When asked why I just said that I was being asked to work an inhuman pace and harassed when I couldn't, HR told me to wait one moment, called up my FT sup who told me I didn't have to listen to a damn thing my PT sup told me and to just keep doing my job (and that I was doing it very well). Whatever, I bought it, kept doing my job. I'm glad I didn't walk out on it, it's still a pain in the ass some days but the benefits are insane, it's part time, and now I can tell my sup to
off whenever I get pissed and that really brightens the mood.
(I could have handled it better, and should have, but I didn't know the grievance process. I would recommend you find your steward and talk to him about it. Best of luck.)