I actually should probably give an explanation more than just my unhinged rant at a feature I've hated for years, if you're going to thank me for one.

From what I understand, when you 'Shutdown' with Fast Startup enabled, it closes many of the programs it would normally close on a log off process, and maaaybe a few more processes beyond that, but it keeps core Windows processes running in RAM.

Then it snapshots that RAM to the drive, probably in a file Windows goes out of its way to be contiguous memory blocks if we're discussing an HDD. Or you'd hope, assuming MS knows what it's doing and I understand what Fast Startup does.

Then it sends the PC into the lowest possible sleep state, but possibly with power still flowing to a keyboard or mouse to allow interacting with those devices to turn the PC back on (despite you having clicked Shutdown). (S4)

Then, on boot-up, the PC "remembers" that this is what it did during its last shut-down process and simply loads this saved core set of processes from the saved RAM snapshot, which means it's loading one singular file into RAM, rather than dozens or hundreds of individual files in the Windows directory.

This speeds up the boot up process by entire seconds if you're on an HDD (insert eye rolling levels of not being impressed here) and maybe a full second if you're on an SDD (if you're lucky) but results in Windows technically continuing to run as if it hasn't been rebooted. You'll even see the "Up time" value in the Performance tab in Task Manager continue to tick away as if it had been running for hours or even days.

An immensely frustrating "feature".

Side-note, not a sysadmin, but if anyone knows of some work... I'm looking.