Open Settings and browse to System > Power & sleep . On the right side of this screen, click Additional power settings to open the Power Options menu on the Control Panel.
Here, click Choose what the power buttons do on the left sidebar. You’ll need to provide administrator permission to change the settings on this page, so click the text at the top of the screen that reads Change settings that are currently unavailable.
Now, untick Turn on fast startup (recommended) and Save Changes to disable this setting.
If you don’t see the Fast Boot option, you don’t have hibernation enabled and thus it won’t show up. To enable hibernation, open an administrator Command Prompt or PowerShell window by right-clicking on the Start button and choosing Command Prompt (Admin) or Windows PowerShell (Admin).
Type the following command to enable it, then try to disable Fast Startup again:
powercfg /hibernate on
Update your Graphics Driver:
Make sure your PC graphic card is updated. The outdated graphic card also can cause the PC to boot slow.
Slow boot time can also be a cause of viruses and malware attack on your computer in various ways, e.g. deleting your files, encrypting your folders, and making your computer running slow. To scan for potential viruses or malware existing on your computer, you can either use Windows Defender or the anti-virus software you installed. If you do not have any third party software please follow the below instruction to perform deep scan with Windows Defender:
Step 1. Go to “Settings” > “Update & Security” > “Windows Security”.
Step 2. Click “Virus & threat protection”.
Step 3. In the “Threat history” section, choose full scan and then click “Scan now” to scan for viruses on your computer.
Step 4. If there are viruses or malware found, follow the onscreen instruction to remove them.