Gatsby is an SSG built around React which provides an amazing set of tools and environment for generating blazing fast web pages. When we talk about speed optimization, nothing can beat static HTML files.
Building with Gatsby results in a fantastic developer experience. It has a huge list of plugins at your disposal which will greatly speed up development.
The final website is served as static HTML and is entirely cachable. It can be fully served from CDN which will result in very fast page loads.