When offering positive constructive feedback, you might say, "Thank you for processing all of the files while Bob did the follow-up calls. Your efficiency is a real value to our team." For negative feedback, clarify your expectations; then ask, "How do you think we can be more efficient next time?"
Constructive feedback should be focused and specific. In positive constructive feedback situations, always express appreciation and remember to be specific. In negative constructive feedback situations, express concern for the welfare of the person in question. Expressing anger or disappointment is counterproductive. Rather, try to create awareness that leads to correction or improvement.