Are those legacy-supporting sites really the slow ones though? Targeting old software usually also means targeting old hardware, so those sites tend to perform

Are those legacy-supporting sites really the slow ones though? Targeting old software usually also means targeting old hardware, so those sites tend to perform reasonably well (and in general, old browsers didn't give developers many options to slow things down). The problem really starts once developers can start ignoring old platforms and (ab)using the full power of a modern browser. The problem isn't any specific library, but the mindset developers use to develop web sites/apps. Bundling 20 dependencies and 10k lines of "compiled" JSX into one giant 5MB uncachable blob that needs to get parsed in its entirety, then shoot some requests off then need to complete, be parsed, then compile some HTML and CSS that then need to again get parsed by the browser and then finally, the the browser's rendering engine gets to start laying things out and drawing the first pixels - that is the problem. The browser is a document viewer, not an application runtime, so you should be giving it documents, not programs. Yes, JavaScript is necessary, but just like you shouldn't be using HTML for layout and CSS for content and graphics (although CSS-only art is impressive!), you shouldn't be using JavaScript for content and styling.