About a month or so ago, my game became impossible to load. I have finally found that the cause of Studio hanging is from any calling of the StarterGui SetCore function to disable BadgeNotificationsActive. The call will always fail. This does not apply to any other acceptable value.
I tested this bug by running the following code in StarterGui in a LocalScript.
local success repeat success = pcall(function() game.StarterGui:SetCore("BadgeNotificationsActive", false) wait() end) until success
I made a place to further identify the bug that can be found here
I have tried it in studio and in-game. It happens in both situations, the call to SetCore will always fail.