It is very important that you deal with HR issues early on where possible. Letting an issue escalate can damage the employer / employee relationship. If it is work performance related then you need to speak with the employee early on, determine what the issue is and try to resolve the issue. Sometimes it can be a simple fix, increased or more focussed training, delegation, or whether the employee has the tools for the job. It may well be the case that the employee may not be forthcoming in informing you of the problem and after all you are not a mind reader so it is important that you have a two way conversation between yourself and the individual to try and determine what the problem is. Only then can you begin to resolve the issue.
However what happens when you have exhausted all possibilities of providing a fix? Well, it may well be a suitability or capability issue and you will need to either seek advice or follow the guidelines that are set out in your HR policies and procedures. However the important message is that you should look to resolve all problematic workplace issues early on because you have an improved chance at resolving whatever the problem is, it can also significantly reduce the chance of litigation providing you have followed procedures correctly.