Windows Boot Manager works very slow

From 15 Windows 10 problems - and how to fix them :

Like Windows 8 before it, Windows 10 uses a hybrid boot to enable fast boot times. When you shut the system down, apps and app processes are terminated, but the Windows kernel itself is hibernated to allow for a faster restart. In theory, it's great, but it seems to still be very slow for some Windows 10 users.

Disable it by :

1. Go to Control Panel / Power Options
2. In the left-hand pane click Choose what the power buttons do.
3. Click Change settings that are currently unavailable
4. Scroll down and un-tick Turn on fast start-up
5. Click Save changes.

This should prevent very slow starts on affected PCs. Some users report that if they subsequently reboot, re-trace their steps and re-enable fast start-up, the problem remains cured.