Darkle the performance impact should be negligible. There is one more database request per page load to fetch the settings value, which is not very different from how other extensions load their settings.
The way it works is actually by creating a second instance of the Flarum router with just the customized routes and calling that router just before Flarum's. The router is essentially one big regular expression being performed on the page URL.
Redirects are an option in the extension. By default original routes will return 404 if no new route was assigned to the same path. But you can optionally enable permanent redirects (useful if you install the extension on an existing forum)
The router of the API and admin routes are not impacted at all, the code only runs for the initial HTML page load. So there might be a slight increase in initial load time, but no change to performance whatsoever once you start browsing inside of the single page app. An other source of negligible performance decrease could be with every API routes that return links, because the extension runs additional logic when the URL generator is resolved.
I believe this is well optimized compared to many other extensions that will have a much worse performance impact.
If you have a very large forum and experience performance issues I would be happy to take a look.
