Windows 10 comes with a feature "Fast Startup" enabled by default. It speeds up the boot process. Fast Startup makes Windows boot very fast.It depends on hibernation. You can disable full hibernation to save disk space but keep Fast Startup enabled in Windows 10.
If you have hibernation enabled in your Windows 10, then a file hiberfil.sys exists in the root of your C: drive. Windows 10 stores there the memory contents when you hibernate your PC. You can shrink the hibernation file in Windows 10 with a single command.