Recently, I spent an afternoon at a 1 million square foot Amazon fulfillment center in Baltimore, where more than 2,500 workers assemble, package and ship orders every day.Why it matters: These jobs are among the most stressful, low-paying and physically taxing in the country — and many of them face ongoing threats from automation. The Baltimore employees make up just 2% of the 125,000 Amazon associates across North America who ensure your Prime packages arrive the next day.How it works: The orders make it through the Baltimore facility with lightning speed. Within four hours after your purchase, your laundry detergent or bag of chips is on its way.Each part of the process is hyper-efficient — and automated where it can be. At the packing station, for example, a line of associates stand for 10-hour shifts, boxing the barrage of orders. A human touch is still necessary to pack the boxes — for now. This week, Amazon made significant headway with new robots that can box up items, reports Reuters. Though they are not nimble enough to handle individual products, these machines are beginning to make Amazon more efficient.The company says the addition of the bots will not eliminate swaths of jobs.My thought bubble: I tried packing two boxes on my visit. Each took me a couple of minutes, but Cliff Knight, a packaging associate with Amazon for more than 2 years, told me he does a box every 5 to 15 seconds.Cliff handed me a container of Tide Pods. I scanned it, and Amazon’s system told me exactly which size box to use and spit out exactly the length of the iconic black and blue Amazon Prime tape I’d need to seal it.The machine then printed a barcode shipping label with customer information. On my first try, I nervously put the completed package onto the final conveyor belt without a label — rendering it pretty much useless. Cliff had to run along the belt to chase it down.I was no doubt the worst packer on the floor during my 2-package stint.The big picture: The stakes are high. 300 workers have been fired over 13 months at the Baltimore warehouse for a lack of productivity, the Verge reported.While this turnover rate is not unusual for the retail and logistics industry, Amazon is vigilant — and always watching its workers on the line.The company has an automated system to track productivity. When a worker is not in the top 25% of performers, conversations with a manager begin and can quickly lead to termination.The bottom line: The stress and speed don't matter. Amazon is getting new applications to work at its warehouses much faster than it can burn through existing employees.After the company raised its fulfillment center minimum wage to $15 an hour, it received 850,000 applications for the seasonal warehouse position, a spokesperson said.Go deeper: Amazon is building a shipping juggernaut