An employee is not performing well, should you go to disciplinary after a few chats with them when there has not been any improvement or performance management?
If the answer does not spring to mind, think how you would like to be treated. If you do not know what is exactly expected of you and there has been a few chats with your manager, you are still struggling, how would you feel going straight to a disciplinary or performance management plan. Unsupported?
As a manager you need to take time out with the employee and see where the problem lies and build up a training plan where the gaps are, teach them some tools to help them improve. You need the facts to support the areas of concern. Speak to the employee to get their input too, they may not have the answers but at least you are working with them. They may not realise they are not performing to the level you want them to be. The fault may lie with the you.
Of course, this should all be documented and regularly assessed to see how they are improving. It maybe the suggested plan may not work, they may need a mentor, shadow someone or be shadowed so be ready to change the it around.
Do not give up on them and go to disciplinary or a formal performance management plan, just think how you would like to be treated. We are all different and respond in different ways.