I was facing this problem for a long time and even Microsoft support guys didn’t know why it was happening after messing with my pc using remote assistant many times. The problem persisted even after I upgraded to Windows 10. Anyway here is the fix to the slow right-click menu in my case:
Remove “HKEY_CLASSES_ROOTDirectoryBackgroundshellexContextMenuHandlersigfxcui” from the registry.
Taken from this link