You can't fix core competency with a stern conversation
When things aren't going well with a new hire, the problem usually falls into one of two categories: competency or engagement. If it's a problem with engagement – their style of collaboration, their communication, their approach – there's a good chance you can fix it with some clear feedback. But if the problem is with core competency – their technical skills – you can't expect to solve that with a stern conversation.
It's true that it might not be entirely clear in the early days of a new hire whether problems fall into one category or the other. But eventually it will be clear, and when it is, you have to act accordingly.
Problems with engagement are often simply due to unstated assumptions. You expect someone to act a certain way, but they don't realize that, so they don't, and you're disappointed. If they knew what was the bother, they'd be able to correct. And you do neither you nor them any favors by sugarcoating or delaying the feedback. This is the essence of radical candor.
But if the problem turns out to be core competency – that they're simply not good enough at what they've been hired to do – you can't feedback your way out of that in any reasonable amount of time. At least not when the problem is with anyone but those in the most junior positions.
If a programmer or a designer or a writer, or anyone else really, spent the last four-five-six years of their career getting their skills to where they are now, it's delusional to think that they can lift them to an entirely new level in just a month or two or even six just because you asked them firmly.
The only time I've occasionally seen it pan out well is if the new hire was merely misclassified on the seniority ladder, but they actually do have solid chops to qualify for a lower level, and thus have a chance to (relatively quickly) grow into the expectations of the position they've been hire into. Say a lead developer who's actually more a senior, but with a couple of challenges could quickly get the last bit.
It's still awkward, though, and one of the reasons why you rarely do anyone a favor by hiring them into a level of seniority that requires a big reach. Better to have them come in where they're solidly qualified, can dazzle against the expectations they can surely best, and then push a promotion from a surge of confidence and goodwill.