It could be caused by particular files in the folder, or by a shell extension.
If it happens in all folders, not just ones with the same types of files, then it is most likely a shell extension.
ShellExView can be used to list and disabled installed shell extensions on the machine to see if any are involved. Details are in this guide:
The guide focuses on crashes, but most of it also applies to slow-downs.
Checking if any process has high CPU usage in Task Manager is also a good idea. If there is high CPU usage, there are further things we can do to diagnose where it's coming from.