I believe the issue is that these software updates, like Time Machine backups, are given a low CPU priority, so they can take a very long time if there are other, higher priority foreground or background processes running simultaneously. It can be sped up with a Terminal command that temporarily disables low priority process throttling: sudo sysctl debug.lowpri_throttle_enabled=0 This change is reversed on reboot or by entering this Terminal command: sudo sysctl debug.lowpri_throttle_enabled=1 I think this is why some people have found success with updating in Safe Mode, because unnecessary processes would be disabled.