When you search the Internet for answers to questions like “Why does Windows Explorer take so long to start” or “Windows Explorer takes too long to start” you’ll find thousands of hits. One I can recommend for a nice overview is here: http://ask-leo.com/why_does_windows_explorer_take_so_long_to_display_sometimes.html
As mentioned in the article the cause usually is that Windows stumbles over a remote connection i.e. a mapped network drive that became invalid or unreachable. HOWEVER, what many overlook is one very important Explorer setting:
Tools -> Folder Options -> View -> Automatically search for network folders and printers
Activating this flag may easily slow down the start of Windows Explorer by 30-40 seconds!