Content Distribution Networks (CDNs) are a class of successful content delivery architectures used by the most popular Web sites to enhance their performance. The basic idea is to address Internet bottleneck issues by replicating and caching the content of the customer Web sites and to serve it from the edge of the network. In this paper we evaluate to what extent the use of a CDN can improve the user-perceived response time. We consider a large set of scenarios with different network conditions and client connections, that have not been examined in previous studies. We found that CDNs can offer significative performance gain in normal network conditions, but the advantage of using CDNs can be reduced by heavy network traffic. Moreover, if CDN usage is not carefully designed, the achieved speedup can be suboptimal.