I prefer forums over slack for serious support and discussion any day. Sure, Slack gives sense that things progress fast, but that is only true when there are right people online, and your question is trivial enough / interesting enough to get attention right away. If neither of those is true, open question gets lost really fast.
I expect new user post approval to become smoother over time, when community grows and stabilizes. Then it most likely has community members who have enough privileges to do such first post approval, reducing time greatly.
In world where most people think everything is in rush and has to happen instantly, forums are nice place to slow down, give a thought on your post and really concentrate what you are saying & doing. 
/me likes 