See the problem is that i explain what i say, you just are not willing to think about it.

Jumping into the dom with querySelectorAll on the whole page to extract tags to use them again to fire javascript functions is inefficient. It will slow down your page if it has enough data in it.

It won't slow down your website, because it is simple, you could write it with the slowest framework in the world and it will work just fine.

Also the reason why websites are slow, has usually nothing to do with the framework in my experience, but with the devs that are proficient in using the framework, but don't know javascript very well and get caught in anti-patterns that make their code unmaintainable and inefficient.