This reminds me of the first time I fired someone. I spent probably a week mentally planning how to do it--thought about how the conversation would go in the car, in the shower, before I went to sleep. The guy had a wife and kids and I was getting ready to put him in the unemployment line. I felt especially guilty because he shouldn't have been hired in the first place; we were desperate at the time. That was a mista…
He kept it very, very short- the rationale being that if someone is expecting it, then they know the reasons why. If they aren't expecting it, then the shock means they're unlikely to listen to anything else you tell them anyway.
If they want feedback at a later point, great. But in that moment, it's easier on both of you to just get it done.