Explorer UI is capable of showing loading animation for folder contents since it was first publicly released. It makes sense not only in context of potentially slow devices, but also because "Folder" is not an "Directory" but an somewhat arbitrary COM object and thus showing contents of folder can lead to a lot of potentially slow operations.

IIRC somewhere along the line of all the Win32 windows versions the loading animation got somewhat broken and explorer window was more likely to get simply unresponsive instead of displaying loading animation, but that is another story.

Folders in Windows can be network locations or internet locations without any guarantees on their access speed, makes sense to show to the user that it's still doing something.

A big part of that loading bar, outside of network ops and slow drives (and remember, today you don't have floppy drive noise to know if it works or got hung), is for things like sorting and getting file icons and other attributes right. This can take non-trivial amount of time.