Fast boot... do you mean Fast startup?
The function in a motherboard bios isn't the same one as windows Fast startup
Fast startup won't run if the PC has lost power before hand. The function relies on PC having power to constantly save a copy of Open drivers + kernel in ram, so that when PC starts up its already half loaded. You better off turning it off completely as then you won't get corrupted files when you unplug it after shutdown. If you have an ssd you don't need it anyway, it only really helps on systems that boot off hdd.
https://www.tenforums.com/tutorials/4189-turn-off-fast-startup-windows-10-a.html
Fast boot after power loss probably just skips a few steps in the normal bios boot process to launch windows faster.