How To Identify Performance Testing Requirements ?

Simple answer it would be specified in the requirements document.
In the event requirements document is silent on performance requirements and you need to come up with requirements, then look at historical data collected from existing online applications that are similar or same as the application being built. There are bunch of tool out there to capture traffic and clicks and times to maintain ongoing applications

OR

Look at competitor sites and capture the performance metrics such as time taken to load first byte, time taken to load page without images, time taken to load the entire page, time taken to load images etc.
Use the competitor metrics as baseline for your performance testing and take it from there.

Performance Testing Requirements is totally depends on client requirements and type of database and amount of data he is using.
Normally organizations which are using windows platform, does use less data, and organization using the Unix platform. It does use very high amount of data.

Performance testing requirement is gathered from the user/client requirements. It comprises of functional as well as non-functional requirement asked by the user and the testing process checks weather the product indulge the requirements or not.