1-2 minutes dark screen before GRUB starts

Hello,

I managed to dual boot Windows 10 and Ubuntu 18.04 (LTS) on my Latitude E7440, but before GRUB starts (and before the Dell logo is displayed) the screen remains about one minute long dark. Then the Dell logo appears and I can choose Windows/Ubuntu with GRUB.

Another thing, when restarting the portable, either from Windows or Ubuntu, with 'Restart' option, the computer starts normally.

I tried removing the battery and holding the alimentation button about 30 seconds, but my problem still persists.

I read blowing on the portable's components could solve the issue, but it didn't affect the machine.

It isn't so embarassing, but I want to get rid of it.

Thanks for answer.