High-performance Web servers are essential to meet the growing demands of the Internet and large-scale intranets. Satisfying these demands requires a thorough understanding of key factors affecting Web server performance. This paper presents empirical analysis illustrating how dynamic and static adaptivity can enhance Web server performance. Two research contributions support this conclusion. First, the paper presents results from a comprehensive empirical study of Web servers (such as Apache, Netscape Enterprise, PHTTPD, Zeus, and JAWS) over high-speed ATM networks. This study illustrates their relative performance and precisely pinpoints the server design choices that cause performance bottlenecks. Once network and disk I/O overheads are reduced to negligible constant factors, the main determinants of Web server performance are its protocol processing path and concurrency strategy. Moreover no single strategy performs optimally for all load conditions and traffic types. Second, we describe the design techniques and optimizations used to develop JAWS, our high-performance adaptive Web server. JAWS is an object-oriented Web server that was explicitly designed to alleviate the performance bottlenecks we identified in existing Web servers. The performance optimizations used in JAWS include adaptive prespawned threading, fixed headers, cached date processing, and file caching. In addition, JAWS uses a novel software architecture that substantially improves its portability and flexibility, relative to other Web servers.