回答先: When I shut down my MacBook Air it shows my desktop wallpaper.I had the same problem with my new Mac book pro. I bought it recently and after reading some articles I found that deleting cache might help, cause I think the cache that has been collected creates a lag or slows the Mac down. So I highly recommend using a third party app cleanmymac 3 but to delete all the accumulated cache you have to buy the app as their is a limit of some mb or gb that can be deleted another way is doing it manually. My situation improved a lot so I'm planning to delete the cache manually as buying the app itself is something I left as option B. And you can surf the internet on how to delete the cache. Moreover, copy all the files before deleting because it may cause some issues and if their is an issue simply paste those files back to cache folders. Try opening all the folder one by one and deleting the cache rather than deleting the whole folder.