In case it was not explicit enough before, it is crucial to know what the server does for your application. It is already key in development, but when you start working on performances, it is a must. This means that you need to know the application good enough to know which part of the server it will use and which implication it will have on your performances.
In other words, you will need to fully understand the use case of your application but also what technology was used to implement it. A simple example is if your application used RESOURCE_LOCAL mode for JPA but you see a lot of JTA use, then you will need to identify why. If you don't have this kind of insight, you will just think the application uses ...
