It also is still general overhead, which does require capable devices and a good internet connection. Something a lot of developers with very capable computers and fast internet connections tend to overlook.

Specifically, if you are targeting a global audience, there are entire geographic regions where the internet is much, much slower and less reliable. So not only are these people experiencing slow load times with packet drops and all that some JavaScript libraries and such might not even load. Which isn't a huge deal if your main content does not rely on JavaScript to load, but of course is if it does require JavaScript.

In addition to that, in these same regions people often access the internet through much cheaper and slower devices.