The default Magento theme - Magento/blank & Magento/luma - and all deritives have been called slow for more than a decade. However, the biggest culprit here is JavaScript. Loki Theme for Luma removes all legacy JavaScript and replaces it with modern JavaScript, including Alpine.js. The result: A perfect Lighthouse score.
We'll come up with demos soon.
Disclaimer: This assumes a plain Magento site with few extensions. Any extension you add on top might require additional effort.
Create a child theme based upon your current Luma theme. This serves as a cross-over theme to move your pages into Loki Theme step-by-step.
Configure the LokiTheme_LumaComponents module for your new theme. This removes all legacy JavaScript and replaces it with new components.
Use the Yireo_ThemeByRoute module to apply the theme only to the checkout, CMS pages and other pages that you wish to convert.
This is open source effort to upgrade Luma to new levels. Many components will still need to be written. Let's collaborate. Or just hire us.