A while ago I did a quick test conversion of 1MM posts which has resulted in a Flarum DB of 1.1GB in size.
Performance is not dramatic but the DB schema that Flarum uses hasn't really been optimised yet. This is probably fairly easily fixed by adding some indices where appropriate. Haven't looked into this yet, first need to find a way to add some kind of debug bar or query logging to the pages. I'm used to Symfony apps, Flarum works quite a bit differently, especially with the JS frontend app.
In my case server times for most pages are between 600 and 1000ms without any optimisation. Not nearly acceptable for our use, but it seems to be on par with the performance of flarum.org at this time.
But track this Github issue:
flarum/core127