I'm sick of being "too slow."

so I've been working at walmart overnight for about two months. the first month i joined, i was constantly being put on a pedastal seemingly for catching on so quickly and how quickly i go across the store to get stuff done. essentially i bust my ass every night because i wanna leave for 7 every morning. i used to work in HBA but they had to move me because one of my coworkers consistently bullies me and is borderline discriminatory towards me, but management won't do anything about it of course. she tries to make me look bad and blames her or others' mistakes on me. anyhow, i've been usually working paper which is about 10 pallets per night. I'm usually done for 5:45-6 and start zoning but I've been called to the office twice now for being "too slow." I know I'm doing at LEAST a case per minute, if not way more for bulk pallets. i will hold multiple cases in my hand and push or carry them where i need to go, damn near running to each aisle. i don't understand what's with the switch-up and what i could possibly be doing wrong. i am head-to-head with the amount of hours that the most efficient workers get that have been working there for years. everyone else gets a couple of aisles and at most 3 hours of freight. would they actually fire me?? they keep threatening that my 90 day period is coming to an end and they can replace me, but truly there is NO way i am able to do all of this and be done at 4am like they want me to. maybe I've been putting in far too much effort and expect the impossible from me, or i need to grow an extra pair of limbs to make 10 pallets into 5 hours of work. help

edit: they also have put me with new hires multiple times with no training. im guessing they expect me to train their employees too all while doing this