Can't enable ACPI on Ubuntu

ACPI is tued off by default on my freshly installed Ubuntu 16.04 and it seems because of that only one CPU core is detected, resulting in a lot of performance issues. Tuing ACPI on grub makes the OS unbootable. Ubuntu would get stuck on a page with no relevant error.

I searched around and it doesn't seem to be a common problem. I would really appreciate any kind of help. Thank you.