Web application today are become more rich and complex. For building such application developers are using Ajax and Web 2.0 technologies. This powerful technologies offer advanced features for building user-friendly and highly interactive web applications that are providing quality end-user experience. Deploying web application is a challenge both in assuring that the functionality will be maintained and in guaranteeing that the functionality will be delivered with an acceptable performance. Performance problems can bring all kind of undesired consequences. For web applications especially in an e-commerce situation, performance testing is crucial. Testing Ajax web applications presents its own set of challenges. The complexity of new technologies, the lack of commonly recognized and accepted standards, and the sheer multitude of emerging frameworks and toolkits make it difficult to test Ajax web application performance. Performance testing is a type of testing that is performed, from one perspective, to determine how fast some abstract of a system performs under a particular workload. The performance of a web application depends on many factors including the network, the end systems, the application response time and most importantly the end user. In this paper we discuss general concepts, practices and tools that lie at the core of performance testing web applications. Performance testing includes monitoring and recording performance levels during regular, low and high stress loads. Performance analysis tools from the open-source such as Apache JMeter, YSlow and Firebug can be used to achieve effective web application performance tests. A case study will be given to demonstrate these tools used in a Ajax web application. In this paper we conclude that performance tools helps software developer to find out bottleneck in performance of the system as well as to choose a good platform for designing web application.