Here are some average numbers I fetched now. I have not logged into nightly.
Nightly has only 1 discussion, whereas discuss has 7000+
These are the response time of each individual request, not the time until the browser starts rendering.
https://nightly.flarum.site/api about 50ms
https://discuss.flarum.org/api (disconnected) 360ms
https://discuss.flarum.org/api (connected) 400-500ms
All info about the forum and the current user. Shouldn't be much impacted by the number of discussions, but if the database is busy it can increase the load time. Each extension also loads its own settings there, so each extension means additional processing.
https://nightly.flarum.site/api/discussions 50-60ms
https://discuss.flarum.org/api/discussions (disconnected) 380ms
https://discuss.flarum.org/api/discussions (connected) 360-400ms
Load time will be directly linked to how many discussions and posts there are on the forum. Extensions can also play a big role by adding new relations.
https://nightly.flarum.site/ 60-150ms
https://discuss.flarum.org/ (disconnected) 560-600ms
https://discuss.flarum.org/ (connected) 640-700ms
The homepage includes the full /api
and /discussions
payload, so it's roughly those two added up, plus the time to resolve the homepage.
https://nightly.flarum.site/404 40ms
https://discuss.flarum.org/404 (connected and disconnected) 210-220ms
This is a control value. Here almost no data should be fetched from the database (only settings and maybe permissions), so the time to answer with a 404 error can be linked to the server performance, but also the number of extension adding routes and middlewares.
I think the "fast" aspect of nightly is mainly down to these things:
The beta/nightly server has almost no traffic. So even if both nightly and discuss were hosted on servers with the exact same specs, I'd still expect nightly to perform better because no one else is using the webserver or database resources.
There's almost no data in the database. A single discussion, probably very few users. A single tag. etc...
Very few extensions. The more extensions, the longer the loading time could be. But not all extensions will affect the same parts of Flarum. Some might slow down the router, other only specific endpoints.
And on the other side...
If you go to the network tab of the browser you'll notice CSS and JS loading is blazing fast on discuss (<20ms), whereas it's comparably very slow on nightly (between 100 and 200ms for the main JS). It's not noticable when browsing because nightly is so fast to load overall. This I think is basically down to Cloudflare. Discuss is using Cloudflare, nightly isn't.