A folder may take too long to open because Windows is processing thumbnails for videos and pictures even if they are not needed for the file types not related to both.
To solve this, go into any folder especially the parent or root that has many folders in it. In this example I select Download folder.
Right-click on any blank surface in the folder and choose Customize this folder.
In the Customize tab, under Optimize this folder for: select General Items and check or tick the box Also apply this template to all subfolders.
Click Ok to save.