The role of a content delivery network (CDN) in web performance is to distribute and deliver web content efficiently to users based on their geographic location. By caching copies of content in multiple locations around the world, CDNs reduce latency and improve load times, enhancing the overall user experience. They also help in balancing traffic, mitigating DDoS attacks, and providing failover redundancy. Using a CDN is essential for websites that require high performance, especially those with global audiences.