What do people think about offering first time criticisms in private? I've had pretty good results with employees by saying something like, "We have you here f

What do people think about offering first time criticisms in private? I've had pretty good results with employees by saying something like, "We have you here for a reason. We know that you're good. So far on this project you've been learning/on cruise control/ramping up/adjusting but we can't afford that now. Now is the time to produce." If that doesn't work, then bring it up in a team meeting while focusing on the task rather than the person.