Philo has been streaming 4 mara/4 shadow and clearing 70s in around 4 mins with 2 people with FnR. So theres a lot of tuning that is necessary unless they plan on just letting people go crazy and go for super high rift numbers. With decent gear it looks like most classes can do 4-5 min 70s solo.

Those rings are absolutely silly though, but it does allow for diversity once they get balanced.