Web Application Performance is one of the most critical non-functional requirements, but without proper planning / knowledge its the hardest nut to crack – be it the requirements definition by the customer or be it’s implementation by the service provider.
There are very few customers who really know how to define there application performance requirements. Most customers are unclear about it – they would have not defined it because they don’t have a clue as to how to go about doing so; and if they have done so - it would just have been from top of there head which most likely will be unrealistic.
This become even a bigger challange when the organization is looking to outsource the project – they like to get the best possible performance but then don’t quite understand the complexities involved. The service provider not always know the application nitigrities, and is thus reluctant to commit on these requirements.. (I’ve myself been there a # of times negotiating and education customer on there performance required)
There are two key ways of going about defining your performance requirement upfront:
First is that to define your unique web application performance requirements – you can develop a working proto type for the key functionalities of your application where performance requirements are most critical. This prototyping approach will help you understand the # that can be achieved. This is the most scientific approach to performance planning.
In case adoption of this is not feasible due to time and resource constraints, one can also look at Historical Data. Anytime you’re looking to revamp or enhance your existing site, you can use the performance #s of your existing site and use is as a baseline to define the performance requirements of your new application.
Apart from these, you can always look at competition web sites with similar functionalities – they can give you a good starting point – what’s the least you want or what you don’t want for sure 
And if you can’t adopt any of these, be a little bit patient and ensure that performance is give in its due amount of attentions in all phase of the project: Requirement Analysis, Design, Development, Testing and deployment. As is it the application performance has to visited and addressed at every stage of the project. In this case you may not be able to define the target # upfront but for sure you’ll be able to achieve the best figures.
