Explorer Very Slow and Nonreactive

"Download Sysinternals' Process Explorer
Run the program with administrator rights (so it can access all the information it requires from the system).
Right click on the process Explorer.exe, and select Properties.
Select the performance tab.
Now open a new Explorer window, and watch the graphs. You should be able to tell whether the problem is i/o access or cpu (I haven't found the memory to be a probable cause for this behavior).
Other tabs like Performance and Threads, should help you to figure out what's abusing the system's resources.

If you reckon the problem could be some corrupt dll or handler attached to the Explorer process, then follow these steps:

Download Sysinternals’ Autoruns (it doesn’t need to be installed).
Uncompress the zip file and run the program autoruns.exe with administrator privileges.
Wait for the program to retrieve all the information, then select the Explorer tab.
Look for any item without a publisher or from a dubious source.
If you find any, disable the item by un-checking the box to the left, then close the application and reboot the system.
Repeat the process with any other dll/handler you think it could be causing the problem. Reboot again after each one (you can also re-enable the items by checking the corresponding box).

Hope that helps."

"I would try Process Monitor from Sysinternals which should allow you to see what sort of file accesses are being performed when you open a file.

As I understand it, Explorer attempts to find icons for each file which sometimes attempts to locate the associated application, (so it shows the excel icon beside a excel file etc). However if there is a application located on a network folder (or on a memory stick) then this can slow it down. And of course if the drive path is not there, eg a memory stick that isn't plugged in or network path it can't find, or if permission to the file is denied, this will slow things down.

Process Monitor will also show you lots of other stuff that goes on when you open a windows, for example, sometimes when explorer attempts to access an application to determine the file icon, this triggers an anti-virus scan.

Note the icons were supposed to be cached, to avoid repeated look ups, but that may not be happening. There is a registry key which controls the size of the cache

Hkey_Local_Machine\Software\Microsoft\Windows\CurrentVersion\Explorer\Max Cached Icons

(Mine is set to 2000, but maybe you can try increasing it).

The cache can also get corrupt, but I never heard of this causing a slowdown, rather it sometimes causes the icons not to be drawn correctly."

"Delete the following value from the registry to disable shared documents:

HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\MyComputer\NameSpace\DelegateFolders\{E211B736-43FD-11D1-9EFB-0000F8757FCD}

References

Get Rid of Shared Documents
Remove Shared Documents folders from My Computer System Folder"

"After following @nik's advice, and then @Leonardo's advice, I discovered one extra possible issue not already mentioned...

Look in the user profile ""Desktop"" folder. If there is a large number of files/folders there, archive (move) as many of those files/folders as possible to a different folder on your computer. The reason being: explorer.exe on Windows 10 seems to iterate over files/folders inside the Desktop folder on launch, and possibly at other times (and more files/folders means it takes longer to process that list).

In my case, I moved 3500 files away from the Desktop folder, into a different folder in the file system, and I noticed a speed improvement of 20 seconds."