This seems like a case of not managing expectations. The following should be clear: 1. We fire fast. 2. We don't want to fire people and will do our best to help you succeed. 3. Here are the bars you need to clear in order to stay with us. (They should be reasonable.) 4. We will provide frequent feedback to let you know where you are.

Not sure about everyone else, but to me it's often obvious who wasn't going to make the cut within the first 1-2 months of their employment.