> they actually understand how HTML, JS, and css work under the hood they would increase their speed massively.

Indeed, Svelte is built directly on web standards so knowing it strengthens your command of core JS and the web-platform itself - not just a silo-specific layer like React.

Also Svelte acts as a compiler: it outputs just clean, highly optimized JavaScript with no virtual DOM, that uses native web-APIs and results in minimal runtime overhead, smaller bundle sizes and overall excellent performance and DX.