My summer job had it's ups and downs. It was my first one too and it sucked when I got told I worked too slow (whether I was preparing food items, or shelving things, or helping with the register). I like to think I was being meticulous. :look:
It wouldn't have been so bad if those same supervisors who took their time to point out how slow I'm doing something didn't disappear randomly when someone needed them. They also didn't address some of the more problematic employees like the cashier who text messaged while there were people on line, or the counter guy who pocketed souvenirs (small ones but still they're expensive).
For what it's worth, I did improve my speed after about a month of practice, and those two got fired just last month (but that could be because they've been cutting back hours and letting people go recently. Something about low ticket sales and needing to recoup losses.)