It is one of my signals of inexperienced management that they believe they can/should take on this sort of challenge.
Of course it's a judgment call. Some things, you cannot fix. But some things, you can, and the cost of doing so is much less than rehiring. Good management means making those judgment calls, not just saying "meh, I don't care about low performers, too much work".
Maybe, just maybe, that's a tiny part of the reason people on my team seem to stay longer than 4 years. Because we don't just treat them as disposable cogs.
And yes, not every situation is salvageable. Alas. Knowing when to stop is why I'm in favor of having a defined process well before you come to the "change, or else" part. That prevents the sad effect that your bar shifts as you collect reports, just because you've got more things to do.
It's a boatload of work. Expect to spend as much time per week as you usually spend on 3-5 people. Then make the call if you can afford to do that. It's buying you a lot in terms of employee goodwill and team morale. (The second one only if you don't neglect the team over it, obviously :)