Hi Paul,
Thank you for posting your query in Microsoft Community forums and providing me an opportunity to help you.
I understand that Windows 8 start up is too slow after shut down or restart and disk active time show 100%.
Let us follow the troubleshooting steps listed below and try to fix this issue, but before we proceed let me know the follow information.
1) What is the make and model of the computer?
2) Do you remember making any changes prior to this issue?
Let us follow the methods given below and check if the issue persists.
Method 1:
Unplug all the devices connected to the computer except keyboard and mouse and try restarting the computer.
Method 2: Perform “System maintenance troubleshooter”
a) Press Windows key + X, Select Control Panel.
b) In the search box, type troubleshooter, and then click
Troubleshooting.
c) Under System and Security,
d) Now click System maintenance.
e) And follow the steps.
Method 3: Let us try booting the computer in safe mode and let us perform a clean boot.
Step 1: Perform Safe Mode
Click the link and follow the steps “Windows Startup Settings (including safe mode)”
http://windows.microsoft.com/en-US/windows-8/windows-startup-settings-including-safe-mode
Check if you face the same issue in safe mode. If the issue doesn’t persists in safe mode, the place the computer in clean boot and check.
Step 2: Perform Clean Boot.
To help troubleshoot error messages and other issues, you can start Windows 8 by using a minimal set of drivers and startup programs. This kind of startup is known as a "clean boot." A clean boot helps eliminate software
conflicts.
http://support.microsoft.com/kb/929135
Important: Reset the computer to start as usual.
Note: After you have finished troubleshooting, follow these steps to boot to normal startup.
a) Press the
‘Windows + R’ key on the keyboard.
b) In the
‘Run’ windows type ’MSCONFIG’ and click ‘Ok’.
c) On the
‘General’ tab, click the ‘Normal Startup’ option, and then click
‘OK’.
d) When you are prompted to restart the computer, click
‘Restart’.
Method 4: Run Microsoft safety scanner.
Download and run Microsoft Safety Scanner from the link to make sure you computer is not infected by virus:
http://www.microsoft.com/security/scanner/en-us/default.aspx
Note: The Microsoft Safety Scanner expires 10 days after being downloaded. To rerun a scan with the latest anti-malware definitions, download and run the Microsoft Safety Scanner again.
Note: Any data files that are infected may only be cleaned by deleting the file entirely, which means there is a potential for data loss.
Also refer to the articles given below.
Ways to improve your PC's performance
http://windows.microsoft.com/en-US/windows-8/improve-pc-performance
Windows hangs or freezes
http://support.microsoft.com/kb/2681286?wa=wsignin1.0#method2
Hope the information helps to fix the issue you had with Windows 8 computer. If you have any further queries, or any queries related to Windows, feel free to post. We will be glad to assist you.