@simon, it looks like it can be smaller than 7KB.
$ curl -Ss https://cdruan.github.io/tw-external-js/ | wc -c 6787
In @cdruan’s html, I don’t see $:/boot/boot.js, so that must be externalized as well. Probably that is the reason for the size difference. I didn’t look into the details to see how tw-external-js accomplishes this.
$:/boot/boot.js
tw-external-js