Performance issues can be due to many factors, and to identify the root cause, you may need to involve multiple groups within the organization. This makes it challenging and, sometimes, even political. Also, performance issues are complex and hard to reproduce. Therefore, it's important to understand the issue clearly, set priorities, and get the appropriate people involved for the analysis.
The very first step is understanding the issue. "We are having performance issues!" is a very broad statement. You need to identify all the symptoms, and these symptoms may help you define the course of action. It's important to ask the right questions, such as:
How many users are affected and in what areas of the business?
Is this a general performance issue or related to specific processes?
Is there a pattern for the issue like particular users and/or times of the day?
Can it be recreated in a test environment? If not, can it consistently be recreated...