It's very typical to import code from other files, but it's also typical to have a minification step that essentially performs what you're saying, compressing the files down into something more optimal. In fact more advanced solutions essentially stream the minium amount to users as needed, and compute as much as possible in the server side.

To be honest, I'd bet a lot than by not utilising larger libraries and their standardised functions, your code has a good chance of running slower. Besides, for the typical computer and network capabilities today, there's a lot of wiggling room.

That said, for absolute tip top of performance (where experience is a trade off) you can find fun things like this, where groups do have to push for the upmost performance.