There are a few elements to the word “performance” that could possibly be a bit confusing here.
From what you’ve said, it sounds like one of your biggest concerns is about the speed of the application. “Performance testing” (also known as “load testing”) is a term that usually relates to how fast things run—page load, database queries, network latency, that kind of thing. These are problems that mostly have technical solutions.
For instance, a good DBA will write queries that only retrieves the rows necessary for a report, as compared to retrieving [I]everything[/I] and then only displaying what’s necessary. Replacing any “SELECT *” in your SQL with more specific criteria, for example, will do wonders for performance. Or a subroutine may be coded inefficiently, looping through every item in an array rather than using a hash to locate the information quicker. And your technical architect should have expertise in setting up your infrastructure (implementing multiple web servers, for example) in order to manage a large volume of traffic.
These are not things that fall under the scope of User Experience. Yes, the user experience is impacted when these things cause problems, and a UX designer may raise these issues for the technical team to address (perhaps as a result of conducting usability testing, and reporting that users found the application to be slow). There also may occasionally be solutions to performance problems that can be implemented by a UX designer (displaying useful feedback during slow page loads, such as a progress bar).
But if it’s tips on how to optimise the performance of your application, you should probably seek advice from a person or community with expertise in the technology you’re using. We’ll only be able to help so much here.