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 searching for Power Options in the Start Menu and running the matching Control Panel applet, then in the left-hand pane click Choose what the power buttons do. Click Change settings that are currently unavailable, scroll down and un-tick Turn on fast start-up, then 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.
If you’re dual-booting between Windows 10 and Windows 7, switching fast start-up off will also fix the problem where Windows 7 checks the disks each time you boot it: With fast start-up enabled, the earlier operating system doesn’t recognize that the disks have been properly shut down by Windows 10.