Temporary added it in-line for testing. I was already in the sub 100ms level but this just puts it over the top! Also updated all admin add/edit/delete/toggle/logout links with "data-no-instant". Pretty easy. Open developer preview and watch the network tab. Pretty neat to watch it prefetch! Thanks for creating this!
ps. Working on adding the license comment. I strip comments at the template parse level (working on that now).
pps. I was using https://developers.google.com/speed/pagespeed/insights/ to debug page speed before. Then working down the list of its suggestions. Scoring 98/100 on mobile and 100/100 on desktop. I ended up inlining some css, most images are converted to base64 and inlined (no extra http calls), heavy cache of db results on the backend, wrote the CMS in Go, using a CDN (with content cache), all to get to sub 100ms page loads. Pretty hilarious when you think about it but it works pretty well.
I'd expect this preload script to remember the pages it's already fetched and not duplicate work unnecessarily. :/
If you really care about speed anyway, you should already have setup your site to max out caching opportunities (Etag, Last-modified, and replying "not-modified" to "if-modified-since" queries) - I'd suggest the author should ensure the script does support caching to the broadest extent possible - hitting your site whenever appropriate.
ps. I do tons of caching for any images and videos though. I know those things never change so have weeks of caching enabled.
I'd be very curious to know what your performance looks like over time, especially as it relates to various improvements that you try out.