What are the key differences between synchronous and asynchronous server architectures, and how do these architectures impact the scalability and performance of web applications?

Synchronous server architectures handle requests sequentially, often waiting for a task to complete before moving on to the next, which can lead to blocking and reduced performance under heavy load. Asynchronous architectures, on the other hand, handle requests concurrently by allowing tasks to run in the background, improving scalability and performance by utilizing non-blocking I/O operations. Asynchronous architectures are better suited for I/O-bound applications and can handle higher concurrency levels, but they require more complex programming models and error handling.