Why is it freezing? Are you doing expensive calculations in your ready functions, or are you just adding a ton of renderable objects to the scene?
If you're adding many visible objects at once it will take some time to add them to GPU memory. You could try setting every object's visibilty to false and then loop over them to unhide them while yielding a frame. Something like this:
func _set_visible_lean(root: Spatial):
for c in root.get_children():
_set_visible_lean(c) # Recurse down tree first
c.visible = true # When at the bottom, start unhiding
yield(get_tree(), "idle_frame") # Wait a frame to allow the animation to update
root.visible = true # Don't forget so unhide parent