There is a feature that lets you see which programs are slowing down your system. To access go to Control Panel - Performance Information and Tools. Then click on Advanced Tools in the navigation panel. You will get a screen that shows you the different tools. If there are any shutdown issues, a message will appear at the top of this screen. Click on it and see what programs creates problems along with recommendations on how to resolve them.
There is some registry tweak that can involve a significant reduction
Once you have backed up the Registry, go to Start and type regedit in the search bar. This will open Registry Editor. Then go to HKEY_LOCAL_MACHINE \ SYSTEM \ CurrentControlSet \ Control. You should see WaitToKillServiceTimeout on the right side. Change the value of 20000 milliseconds (20 seconds) to 5000 milliseconds (5 seconds). Do not choose a lower value because services need time to shut down properly. This tweak should make your system shutdown faster. Enjoy.