Employees rarely hear everything you say, particularly when they are ill at ease or under stress, which is common during a performance review. To confirm that your message got through to the employee, circle back on the conversation. For example, "Tom, a few weeks ago we had a performance review discussion. Tell me something - what do you remember from that discussion?" What is the #1 thing you want to have stuck in Tom's memory? Whatever it is, that's your core message.