Ubuntu GNOME: Grey screen and very slow boot

CLI method

sudo nano /etc/default/grub

And change GRUB_TIMEOUT=10 to GRUB_TIMEOUT=1

sudo update-grub

GUI method (mostly)

Install Grub Customizer

To do that, Open terminal by pressing Ctrl+alt+T, and enter the following commands one by one

sudo add-apt-repository ppa:danielrichter2007/grub-customizer
sudo apt-get update
sudo apt-get install grub-customizer

After installing it open it by this command

grub-customizer

it might ask you for your password

1. Go to General Settings tab
2. Check Show menu option under visibility (if you want Grub Screen to be visible)
3. Change the value of Boot default entry after to be less than 10 Seconds (I've made it 1 Second)
4. After you're done click on Save button in upper left corner

Try to reboot your system now, and see if the problem is gone

Theme might not look like this

More info

How to edit files in a terminal with nano?