Poor performance on the Internet leads to dissatisfied users, and dissatisfied users may leave a web site and never return again. Thus, load testing implemented beforehand and corresponding measures taken to make your response time less than 10 seconds can prevent these losses.
Company expenditures on a Web site’s support should be adequate to site’s expected traffic. Ideally, you want to spend enough, and no more, allocating resources where they’ll generate the most benefit. For example, you don’t need to upgrade your Web servers if customers experience most delays in the database server or in some scripts. Thus, to maximize your ROI, you should determine when and how to upgrade IT infrastructure.
Web testing can measure performance of your web system and its components allowing you to invest with maximum effectiveness.
Load testing is a type of testing where realistic (or hyper-realistic) workloads are specified and simulated. The load testing goal is to determine the maximum time (from a usability and customer experience standpoint, not a technical one) it should take for all page elements to load.
The goal of stress testing is to estimate the maximum load that your web server can support. The point is that most performance issues arise only when the server is stressed with a high user load, because many important bugs simply do not surface unless you deal with very large entities such as thousands of users, thousands of mail server mailboxes, multi-gigabyte tables in databases, deep file/directory hierarchies on file systems, etc.