I will always be thankful to a former boss of mine, who took the lead on firing a developer that just wasn't up to scratch, and asked me to be present when it happened.

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.