Windows 10 File Explorer: "Working on it ..."

Open Folder Options > Set "Open File Explorer" to "This PC". Try WinKey + E now. If it opens fine, then the problem is with Quick access cache, which can be cleared by deleting the file f01b4d95cf55d32a.automaticDestinations-ms from the following directory, using Command Prompt.

%AppData%\Microsoft\Windows\Recent\AutomaticDestinations

Note: The above procedure clears your Quick access links. You'll have to manually add the links again.

Short version, the Windows Search service and the Windows Index are having issues.

Launch Control Panel and launch Indexing Options.

Click the Advanced button and you should see an error saying that there is no index.

Click the button to Rebuild the Index and give it 10 seconds and you’re done !!!!

In my case, a couple of folder locations on remote file systems had found their way into the quick-access cache. These remote systems had become inaccessible, so my Windows Explorer was waiting for them to time out before rendering.

Deleting them from the Quick Access panel did the trick for me.

Another thing you can do, especially if you have lots of filetypes Microsoft recognizes in your download folder (stuff like jpg, docx, xlsx, etc):

Right click on your Downloads folder and choose Properties. Click the Customize tab. Choose Optimize this Folder for .... General Items. Click "also apply this setting to subfolders." Click OK.

Repeat for your Documents folder.

This will let the Windows Explorer handle your files without trying to generate snapshot images for your files.