there’s some kind of bug with the grub in 18.04. no matter what you put in the timeout of the grub file, it always takes 30 seconds to boot. do the below. that should get your what you want.
sudo nano /etc/default/grub
#add the below line to your Grub
GRUB_RECORDFAIL_TIMEOUT=5
Ctrl-o
Ctrl-x
sudo update-grub
sudo grub-install