These three dominate the lightweight theme market. All are marketed as fast and customizable. All three have a generous free version and a premium version around 50 to 60 USD per year. So what actually separates them?
GeneratePress
The thinnest of the three. Under 30 KB without add-ons. The code is extremely clean. If performance is the absolute highest priority and you don't need a visual header/footer builder, GeneratePress wins. The premium version (59 USD) gives you a site library and more customizer options, but the interface is still more "configuration" than "design".
Astra
Most popular with over one million active installations. Good WooCommerce integration and a large collection of Starter Templates that work with Elementor, Gutenberg, and Beaver Builder. The downside: the free version sometimes shows upgrade prompts in admin that can be annoying.
Astra Pro (47 USD per year) unlocks header/footer builder, mega menus, and advanced layout options. It's the most versatile option for agencies needing quick setup.
Kadence
Newest of the three and feels the most modern. Header and footer builder already in the free version, which the others lack. The Kadence Blocks plugin adds advanced Gutenberg blocks. If you're building block editor-based sites without a page builder plugin, Kadence is strongest.
Summary
Pure performance: GeneratePress. E-commerce with page builder: Astra. Modern block-based design: Kadence. None of them is a bad choice. It comes down to which workflow and requirements the project has.