Fast startup in Windows 11 is a feature that helps your PC start up faster after a shutdown by saving the system state to a hibernation file. To enable it, go to Control Panel > System and Security > Power Options, and click on 'Choose what the power buttons do'. Then click on 'Change settings that are currently unavailable' and check the box for 'Turn on fast startup (recommended)'. Note that while fast startup can significantly reduce boot times, it may also cause issues with dual-boot systems or when accessing BIOS settings.