When a user selects an available time slot, the page hangs and the spinning circle just spins forever. I have seen a similar problem on your support forums, but haven't found a solution.
From reading another post about this problem, I deactivated all GCal integration in the Settings page and the problem still happens.
I will say that it doesn't happen all the time, so it is difficult to troubleshoot. However, this is a big problem.