[ubuntu] I've install 16.04 LTS with Fast Boot and Secure boot enabled. Question...

There are two different settings. Windows & UEFI.

Windows has its fast start up or always on hibeation. If you only have Ubuntu that does not matter.

But UEFI has fast boot which is not related to Windows fast start up.
UEFI fast boot assumes system is configured exactly as it was before rebooting. It can speed boot for both Ubuntu & Windows. But if hardware or configuration otherwise changes, then you may need to go into UEFI to change settings. And it can be so quick you have no time to press del for f2 key to get into UEFI.

Often cold boot uses normal not fast boot.
With my motherboard, I had fast boot on/off settings for both cold boot & warm (reboot). And a setting for delay. I set delay to 3 seconds so if quick I can get into UEFI.
If system boots to grub, last entry in grub menu should also take you to UEFI. 'System setup'

If no issues booting with Secure Boot on, then it is fine. Grub does not normally let you boot Windows from grub menu if Secure boot is on. But if no Windows not an issue. Some systems also are not friendly to anything other than Windows and they may need secure boot off.