With the increasing use of geographical data in real-world applications, Geographic Information Systems (GISs) have recently emerged as a fruitful area for research. Nowadays, a GIS can be combined with World Wide Web (WWW) techniques to provide information to a multitude of users. A high-performance web-based GIS, called TerraFly, has been developed in order to provide web-based GIS accesses to the general public. The design of TerraFly considers three major aspects including system architecture, data structures, and networking. The system architecture utilizes the existing resources to achieve maximum performance by using the "Internally Distributed Multithreading (IDMT)" technique. A spatial access method, semantic R-trees, is used to search an object based on both spatial and semantic information. System performance results are presented and analyzed. Reducing network traffic to achieve faster response to users is also discussed.