It depends on when the "right-click menu" slows down as to which "Context Menu Handlers" are to blame. As you may have noticed, your right-click menu changes based on what you right-click. The following list corresponds to what you click and what appears in the menu.
Description - Registry Key
Files - HKCR *\shellex\contextmenuhandlers
Files and file folders - HKCR\AllFileSystemObjects\shellex\contextmenuhandlers
Folders - HKCR\Folder\shellex\contextmenuhandlers
File Folders - HKCR\Directory\shellex\contextmenuhandlers
File class - HKCR\\shellex\contextmenuhandlers
Desktop - HKCR\Directory\Background\shellex\ContextMenuHandlers
But you will probably find it easier to use an editor designed to modify these settings rather than edit the registry itself. One useful editor is shexview, by Nirsoft.
You'll want to sort the column "Type" or "File extensions".
Alternatively, autoruns.exe will allow you edit this information as well.
This app is better organized in that it will clump the handlers based on the registry location (the table posted above).
It allows you to edit much more than just ContextMenuHandlers. The pertinent tab you are interested in is "Explorer". Scroll down until you see the correct poorly behaving registry location, and deselect them one by one until the problem goes away.
Note: Both programs are portable (they needn't be installed). Keep them both in a flash drive and help out others.