This problem has haunted me for a while. Eventually the context menu of the Windows Explorer is slow to load or even completely lock. The solution that I was using was restart the computer, What is somewhat uncomfortable or even impossible when there are tasks that cannot be stopped running. The problem is occurring in the Windows 10 For, but probably affects all versions of Windows.
Today finally I decided to search for the guilty. With the aid of the CCleaner It is possible to check what are the items in the context menu and disable them as a group or individually. In this way it is possible to go by disabling the suspects or even use trial and error to find the culprit. In my case, the culprit was the Adobe Acrobat ContextMenu .... located in C:\Program Files (x86)\AdobeAcrobat DCAcrobat ElementsContextMenuShim64.dll. To disable it, the problem is gone, without that I needed to restart the system.
The CCleaner lets help rehabilitate the disabled items with the same ease, so if the misbehaving item for something useful, You can re-enable it after you restart the computer or take any other measure that do get it working.