Every file? You have one .js file per project, if you're like me. So just throwing those two lines in the top and never having to worry about it ever again seems like a nice option.
In terms of Javascript, as little as I can possibly get away with. The web stuff that I do is mostly CRUD type apps, which can be done entirely server side. The Javascript is only where it make the user experience better, so basic form help or to do a modal, things like that.
Better KISS and fit all of JS in a single script.
which costs time
When you go to a site like reddit.com which uses HTTP/2, you see a gazillion of requests for JS files, many starting at the same time and ending at the same time.
I think that is because newer HTTP versions can put multiple requests in the same data packet. And even HTTP/3 is now supported by over 90% of browsers.