Joining the MUCs makes the UI lag
When experiencing #9334 (closed) the UI gets really laggy. It looks like the Python thread responsible for joining the MUC starves the UI thread, probably due to the fact that they share the same OS thread.
This issue makes it really hard to disconnect from the MUC if it appears to trigger #9334 (closed). Filing it as a separate bug because you can always make a MUC big enough to DOS the UI.