Several users filed bug reports on the delay between seeing "loading grub" and seeing the menu. In some cases, the reason was because the computer's BIOS was first looking at a device other than the one GRUB was on. Making sure GRUB 2 was installed on the same device as the BIOS was first looking solved the delay problems.
Originally Posted by mithrandir77
I don't believe the 30_os-prober script is responsible for the delay during boot. You can check by disabling it and seeing if the delay remains:
If you remove the executable bit "update-grub" will not search for your OSs/kernels on other than your system partition. Thus it will not see your other OSs or put them in the menu.
sudo chmod -x /etc/grub.d/30_os-prober
If they do not change often, you could move them to a custom menu, such as 40_custom. This would allow you to leave 30_os-prober off but still have the items on the menu. It would speed up "update-grub" at the cost of not updating the information on other installed OSs.