Do you have a custom anti-virus program you can uninstall? It just seems incredibly suspect that this one folder in particular has this problem if it's not related to an anti-virus program or system process in some way.
I had a thought. Is it possible that a service like the Windows Search service is being shut down constantly and the system is constantly having to start it back up? Have you fiddled with some kind of service script or service modifying tool?
Can you try to move the files/folders to other location/folder and see if the behavior still ocurs? And if it doesn't ocurr anymore, then try to move back some files to see when it stars occurring again.
I believe I didn't use that kind of software in this PC. I'll try to download the lastest Windows 10 ISO and perform an in-place upgrade to see if it fix the issue.
Another possible reason for slowdowns can be third-party shell extensions and handlers. For example, some codec packs can install handlers for thumbnail previews, and they are known to be problematic, sometimes.
It could be a scheduled task that keeps shutting down a service or a program that is auto-shutting down services that it doesn't think are important. Again, this is just a hunch. It could also be something else entirely. It's really hard to tell what this is because I've never ran into this problem myself.
@antonio8909, Carlos Detweiller has made a good point. A third-party shell extension could be causing this delay. I'd look into this...
After checking all the possible issues, I don't seem to find the solution. I guess I need to schedule a new install of Windows in the near future... Thanks to all people who tried to fix it!
@antonio8909 If you really haven't found anything, there is one option that can help (I don't know if anyone has already suggested it or not, haven't read all posts). This is because you probably have the thumbnails enabled in the preview of allowed folders. (It is in folder options) Disable it for all folders and subfolders and only allow icons.
The thumbnails have never been a problem for the load process at any folder on my PC. My download folder is almost empty now and the problem is still there.
Unfortunately, there is no way to find which computer it is, what processor it has, what it means SSD (which - company, NVMe 2.x, NVMe 3.x or regular SSD, brand, how old, how often it is trimmed, how many partitions, how large is primary partition, what other partitions there are, whether the disk is GPT or MBR), how the page file have been configured, what folders are indexed etc. Maybe this folder is in use while you're trying to open it? That is, another process regularly changes the contents of a folder or does something with the files in that folder. Do you open the folder as a new process or do you open all operations as the same process?
I had same problem even on fresh install but did have large downloads folder on other drive it was virus checker that was slowing it down.
This is the same thing with what I was saying earlier about running dism with the folder C:\Windows\servicing\Packages opened in Explorer which keeps scanning it while the contents is changed. I think this is the best guess of the issue for the OP if indexing was disabled for that specific folder.
Try to configure your browser (all of them which are installed) to ask where to download files instead of keeping the default to use Downloads folder. It may be that a browser related process tries to access the folder at the same time with explorer and cause issues. There are free tools from Microsoft (Sysinternals) like Process Monitor and Process Explorer which can assist with diagnosing.
Yes I would say this is your problem, You could probably speed it up by using some type of NAS software like FreeNAS or software that will sync the folder locally and not always reading the contents of the folder using your internet connection.