You can try looking in the event viewer to see if there are any errors letting you know why the slowdown in shutting down. Another thing you can do is do the following and we can see if you are having any malware issues that need to be addressed.
Please post the malwarebytes log so we can make sure you are using the latest database and then also post a hijackthis log.
Download the HijackThis installer from
here.
Run the installer and choose
Install, indicating that you accept the licence agreement. The installer will place a shortcut on your desktop and launch HijackThis.
Click
Do a system scan and save a logfile
Most of what HijackThis lists will be harmless or even essential, don't fix anything yet.
Post the logfile that HijackThis produces along with the Malwarebytes Anti-Malware log