So to cover some points here.
The fact the software is classed as beta is absolutely relevant. There as big warnings on their own website telling people not to use it in production, the fact someone decided thst was ever a good Idea is super stupid.
Extentions sound great but for core functionality are stupid. There's no easy way to update all of them, and every time you do version upgrades things break, had exactly that happen in the last upgrade and had to uninstall and reinstall most of the plug-ins. Basic core functionality should be able to be toggled yes but also part of the core stack. The fact I have to downloads third party add-ons to do things like private messaging polling and splitting threads, many of them lacking the rich features of other software.
On that, the error handling is shite, I've tried to enable the private messaging add on in multiple versions and there is no detailed error to tell me wtf is going on.
In terms of this forums specific implementation, it's literally an install of flarum fresh out the box. Nothing weird or special, so other than potentially the current now legacy server that it is hosted on not being as powerful as their own forums, I couldn't tell you why we get poor performance. I think a lot is geographic because it runs pretty well in the UK here.
Permissions are a major issue and do impact users, it's impossible to effectively setup permissions using the system they've developed and it means we end up with weird behaviour in order to try to deal with other stupid shit like not having a structure to tags with multiple levels.
Were moving to new software soon because flarum ultimately never met the requirements this community have... And isn't fit for purpose... We need software that is actually supported and remedies a lot of the issues in this thread and previous ones, flarum might be great software in the future but right now it lacks a lot of fairly important features for us.