Do you have an employee whose performance you're unhappy with? Tell them.
Do you have an employee who you strongly suspect isn't going to work on in the long-term? Tell them.
All too often, managers avoid being candid with employees about concerns over performance or fit. They want to avoid a difficult conversation, or they don't believe the person can fix the problem, or they're hoping they can ignore it a little longer.
This is horribly unfair to the employee, who deserves the chance to know about the issues, and it's unfair to your company, which has hired you to, in part, address employee problems head-on.
Yes, a conversation about performance problems isn't pleasant. It sucks for anyone on the receiving end, and it sucks for the manager who has to deliver it. But it is far, far worse to be an employee whose boss doesn't care enough to speak candidly with her about areas in which she needs to improve in order to do well.
Even if you're convinced such a conversation would be fruitless and the employee can't change, she deserves to know. She deserves to know because maybe you're underestimating her, or maybe it would be useful for her to understand the ways in which she's a bad fit for this work, or maybe she just deserves a chance to see the writing on the wall so she can start looking for other positions.
The worst thing you can do when you're unhappy with an employee is stay quiet. Tell the person, and tell them now.