CircleCI offers a range of optimization features designed to enhance build speed and productivity for developers working on both private and public projects. Key capabilities include dependency caching to speed up builds by storing project dependencies, workspaces for sharing files between jobs, and reusing Docker images to avoid redundant builds. Developers can also create custom Docker image executors for specific requirements, utilize test splitting and parallelism to reduce test suite times, and configure resources to meet varying workload demands. Additional features like Docker layer caching and reusable configuration with orbs further streamline workflows, while test summaries and insights provide valuable data on project performance. CircleCI also supports artifacting for external access to build files and allows SSH access for live debugging. Most of these features are free, with only a few marked as premium, aiming to provide a comprehensive and flexible CI/CD platform for developers.