If you are going to write the Performance test cases, you required a different mind set to do that.
As a performance Tester you don't require knowledge of application You have only one mid set
i.e the performance of the application How can the application meet the performance targets
In order to test the performance you have to keep in mind the four things
Basic scenarios you have take care to write performance test case
Cover most of the scenarios of end-user inputs which represent the load on application and see the behavior how the application respond on to that.
You have to keep in mind the business functionality which has to covered all of them.
Monitor the actions by user in the steps so you know how much time is taken to perform the one action.
Keep the single test case for particular load or you can say independent load scenarios.
Prepare the test case that should be reproducible. Means its should be like you can perform the same test case many times. Because in some scenarios like : on-line transition commit only for one time so have to keep in mind how the action are going to effect the DB.
Keep in mind all the rules of inputs your application needed.
Track the Time Of Load your going to give your system. It will keep track where your system going to be crashed.
Last but not lest, You have to write the test cases like you are going to test the performance in real user environment. Like logged-in, logged-out, keep the user logged-in for while these kind of scenarios help you to identify the sessions issues.
These above are are some basic things you have take care. While in some more deep scenarios you have know i.e what kind of network you are using with the machine configuration with how many responding systems for that data you are sending or retrieving.