I think this is the clue. Autosaving happens roughly every five minutes, and an autosave can take this long for larger models. That’s why your delay after exporting takes less time.
If you were manipulating your view with the mouse this will never happen. OS X pays attention to mouse events and will not initiate an autosave while you are using your mouse. I confirmed that the 3DConnexion device does not work the same way. An autosave can happen while the 3DConnexion device is active, and the autosave will not be deferred.
I’ve added a test for 3DConnexion activity and will defer a pending autosave until the 3DConnexion device is inactive. This will be in the next RhinoWIP and in Rhinoceros 5.1.