Gatsby leverages JavaScript to enhance website interactivity and performance, even though its sites can function without it by rendering pages as plain HTML. JavaScript bundles in Gatsby allow for easy interactivity through React, providing a fluid transition between pages and reducing the need for full page reloads. While too much JavaScript can impact performance negatively, Gatsby optimizes the use of JavaScript by preloading necessary files and resizing images for efficient bandwidth usage. Future enhancements, like React's partial hydration, aim to further improve performance by making only critical parts of a page interactive initially. Accessibility remains a crucial focus for Gatsby, which uses ARIA live regions to announce page changes, although challenges persist in ensuring complete accessibility for Single-Page Apps. Gatsby's approach combines the speed of pre-rendered HTML with the dynamic capabilities of Single-Page Apps, and ongoing developments in the React and JavaScript ecosystem are expected to bring further improvements.