Hari it doesn't speed up the website under normal usage. It only makes the first request after a cache clear be as fast as regular requests.
It's intended to be used in production for servers that use continuous deployment to update, enable or disable extensions. But for manually managed forums, it doesn't really provide any benefit.
By default all assets are re-compiled when a first user visits the website after a cache clear. When enabling/updating extensions via the UI the first user is usually the admin so the performance hit isn't an issue. But with continuous deployment the first hit could be a regular user and it's poor user experience if they need to wait seconds for the page to load while the assets are being re-compiled in the background.
Theoretically you could replace this extension with a script that uses curl to load the index and admin pages. It would also cause the assets to be re-compiled immediately without an actual user having to visit the website.
I have found this extension to also be helpful if you have massive amounts of javascript and less, because the webserver process could run out of memory or execution time while compiling them. This extension allows the compilation process to run via PHP CLI where limits can be raised or removed.
The script becomes even more useful (if not required) if you have multiple nodes answering HTTP requests for your forum, because then each node would have to compile the assets on the first user visit, which would cause extreme slowness right after a new deployment for many users. But I don't think many Flarum installations use this kind of infrastructure at this time.
