I frequently used to get 35 or more items per minute back when I was a checker. The key is to use both hands in a swimming motion. That means grab the first item with your left hand. As you pass the item over the scanner, grab the next item with your right hand. When you scan the item with your right hand, your left hand should already be reaching for the next item and so on. Most new checkers, and some experienced ones too, will grab an item with their right hand, scan it, and then pass the item to their left hand and send it down the belt. When you use both hands in a swimming motion, you scan and send the item down the belt in one fluid motion.
Manager can't request a cashier to do more as long as you are making your minimums. Kroger thank god is not an ISCE-type environment and it sounds like the manager in question had that type of background before they came to Kroger. Kroger told us in training they cut us some slack. This is not like Office Depot where anything less than 100 percent is considered a fail. I would let the union know about this.