When you’re opening up folders you’ve previously opened, Windows should be able to display the thumbnail icons very quickly, as it stores the images it uses on your hard drive.
However, I’ve seen the problem David describes. I’ve opened a lot of the same folders over and over and have experienced massive delays in the rendering of the thumbnails.

Like David, I have a lot of PDF files and JPEGs, and without the thumbnail I can’t do anything with the files until I can see what I’m working with.
It appears that Windows deletes the cache, or deletes part of it. Here’s how to prevent Windows from ever deleting anything in the thumbnail cache folder ever again. 
Step 1: Open File Explorer. From the main menu select View. In the menu ribbon that appears, find the checkbox for Hidden Items toward the right side, and check that box.
Next, move your cursor to File Explorer’s text bar. In the text bar, type this file path, substituting your Windows user name for “accountname” where 
C:\Users\accountname\Appdata\Local\Microsoft\Windows\Explorer.
Now you’ve arrived at the location of your thumbnail cache. Next, select Properties.
Step 2: In Explorer Properties, go to Permissions for SYSTEM and click Advanced.
Step 3: In Advanced Security Settings for Explorer click Add to add a new permission.
Step 4: Click Select a Principal at the top and type SYSTEM. You can click Check Namesand it’ll make sure your name is correct.
Step 5: Select Deny in the drop-down menu at the top (leaving Applies to this folder, subfolders, and files as-is). Click Show advanced permissions on the right and check these two; uncheck the rest. Click OK. Reboot just for good measure.

Make sure to select “deny” here.
You’re done! Windows no longer has the power to delete anything in that folder, so you should be all set.