Flarum is a Single Page Application, so I believe those libraries don't add anything useful there. Worse, by downloading a copy of linked pages that will never be used, they consume resources and bandwidth in the browser. So while it shouldn't slow the forum down, I don't think it can improve anything with that strategy.
They might be useful if you have many links to a non-flarum non-SPA application on the same subdomain (for example Flarum in subdirectory, and wordpress at /
).
What would be useful, would be to use a similar strategy to those libraries, but instead of pre-fetching the page, it should pre-fetch the API requests. Such a library would have to be custom written for Flarum as a "drop-in" library would have no way of knowing which API requests will be used when clicking a particular link.