Understanding Windows 8 Fast Startup: How It Works and When to Disable It

Problem: Slow boot after a full shutdown

When you power off a Windows 8 PC and then turn it back on, the system goes through a full cold boot: firmware initialization, kernel load, driver enumeration, and user session creation. On slower storage this can add noticeable seconds to the time you see the login screen.

How Fast Startup works

Fast Startup is a hybrid shutdown that logs off user sessions, saves the kernel session and loaded device drivers to the hibernation file (hiberfil.sys), then powers off the machine. On the next power‑on the firmware runs, the loader reads the saved kernel state from hiberfil.sys, and the user session is restored, skipping much of the initialization that a cold boot repeats.

Enabling and verifying Fast Startup

The feature is on by default on hardware that supports hibernation, but you can confirm or enable it manually.

1. Open an elevated command prompt (right‑click → Run as administrator).
2. Ensure hibernation is available:

   powercfg /h on

3. Turn on Fast Startup:

   powercfg /setacvalueindex SCHEME_CURRENT SUB_SLEEP HIBERNATENABLED 1

4. Verify:

   powercfg /a

   Look for “Fast Startup” in the list of available sleep states.

Alternatively, check the registry value HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\Power\HiberbootEnabled; a DWORD of 1 means the feature is active.

Measuring the impact

To see the difference, you can capture a boot trace before and after toggling the feature using the Windows Performance Toolkit.

1. Start a trace:

   wpr -start Boot

2. Perform a normal shutdown then power on the PC.
3. Stop the trace and save it:

   wpr -stop Boot boot.etl

4. Open boot.etl in Windows Performance Analyzer and examine the BIOS‑to‑login interval.

Repeat the steps with Fast Startup disabled (powercfg /setacvalueindex SCHEME_CURRENT SUB_SLEEP HIBERNATENABLED 0) and compare the intervals. The trace will show a shorter interval when Fast Startup is enabled, reflecting the saved kernel resume.

Trade‑offs and limitations

• Hibernation file size: hiberfil.sys can occupy several gigabytes (roughly the size of RAM). On small SSDs or embedded devices this may be undesirable.
• Firmware or driver changes: Because the kernel state is saved, updates that require a full re‑initialization (BIOS flash, certain low‑level driver upgrades) are not applied until a full shutdown is performed. A simple restart does not clear the hibernation state.
• Dual‑boot and disk integrity: When Fast Startup leaves the NTFS volume in a hybrid state, some Linux mounts may see the filesystem as dirty and mount it read‑only or refuse to mount.
• Interaction with encryption: If the system drive is not BitLocker‑protected, hiberfil.sys is stored in plain text; enabling BitLocker on the OS drive will encrypt it automatically.

When to disable Fast Startup

Consider turning the feature off if you:

• Frequently update BIOS or firmware and need the changes to take effect immediately.
• Run a dual‑boot setup with Linux and observe read‑only mounts of the Windows partition.
• Have a storage‑constrained device where the multi‑gigabyte hibernation file impacts usable space.
• Need a clean boot state for troubleshooting tools that expect a full initialization (e.g., certain WinRE scenarios).

To disable, repeat the enable steps but set the value to 0:

powercfg /setacvalueindex SCHEME_CURRENT SUB_SLEEP HIBERNATENABLED 0

Then run powercfg /a to confirm Fast Startup no longer appears.

Closing

Fast Startup offers a measurable boot‑time improvement by saving the kernel session to hiberfil.sys and resuming it on the next power‑on. The gain is most noticeable on slower hard drives, while the trade‑offs involve disk space, firmware update visibility, and dual‑boot compatibility. By checking the powercfg output, measuring a boot trace, and weighing the limitations against your workflow, you can decide whether to keep the feature enabled or disable it for a cleaner boot state.