That said, even a small bundle, written poorly or with poor library choices can result in the main thread being pegged for a long time in compilation or function call times. It’s important to holistically measure and understand where our real bottlenecks are.
But, libraries like jQuery can take quite a long time to parse, particularly on older mobile devices. Vanilla JS, with zero dependencies, still needs to be parsed, but effectively runs as soon as it hits the browser.
In my own experience, switching to vanilla JS from jQuery has had a huge impact on the perceived performance of the sites I build.