How does Windows Fast Startup Work?

Fast Startup is a Windows feature that allows you to boot your computer in a few seconds rather than a minute. Rather than going through the cold boot path, Fast Startup uses a minimal hiberfile to resume the system. When the feature is enabled, selecting “Shutdown” in the Windows UI doesn’t actually shutdown the system. Instead, it closes all user applications, logs the current user out, and then creates a hiberfile. Because this hiberfile only includes the kernel, device drivers and a subset of applications, it is small and can be reloaded quickly. 

Alternatively, the cold boot path requires loading the kernel and drivers from disk, initializing the kernel and drivers, and launching various user mode applications. This can be especially slow on computers that use spinning hard drives.