Sites that only use vanilla JS can still be heavy, too. I think the underlying message is to do the heavy processing on the server side, and keep the client side relatively light.
This is also why frameworks/libraries like Astro and htmx are becoming more popular. Both of them focus on having minimal frontend JS. htmx effectively reverts back to a pattern that was somewhat common 20 years ago - a small amount of reusable JavaScript to handle common use cases, that hits the server and inserts its response HTML somewhere on the page. I was a web developer back then so it's been interesting to see old patterns come back.