It’s the amount of users and servers in the room. When you join it on a new server it needs to process and verify the state of the room and agree it with all the servers. As the state is massive in this room it can take a very long time to join it.
Sliding sync doesn’t make joining Matrix HQ any faster. This is depending on your hardware and whether or not you have any workers configured. For most users it isn’t worth joining that room anyway.
In my experience, pipewire is leagues ahead of pulseaudio and just works. All the issues I had with pulseaudio are resolved with pipewire and it sounds better as well with the default settings.
Doesn’t look like they bothered to update their macOS client to 64bit. Currently the game is unplayable on new versions of macOS.