Turborepo's speed comes primarily from two core concepts: intelligent caching and optimized parallel execution. For caching, Turborepo uses content-aware hashing. Instead of simply looking at file timestamps, it generates a unique hash based on the content of the project's inputs (source files, dependencies, configuration files). If Turborepo detects that the inputs for a particular task haven't changed since the last time it ran, it won't re-execute the task. Instead, it will instantly restore the output from its cache, saving valuable time. This caching happens both locally and can be extended to remote caches.

Parallel execution means Turborepo can run independent tasks simultaneously across multiple CPU cores. It builds a dependency graph of your tasks and executes them in the most efficient order, maximizing resource utilization. If 'project-A' doesn't depend on 'project-B' for a 'build' task, Turborepo will run their respective build tasks concurrently. This combination of avoiding redundant work through caching and maximizing concurrent execution dramatically reduces overall build and test times in complex monorepos, providing near-instant feedback.