NAS extremely slow - only when browsing folders

This problem first occurred about a month ago (Latest firmwae update???). My hope that it would go away as suddenly as it came up vanished.

I have this device for several years now and generally it works just fine. But whenever windows explorer is supposed to show the contents of a different folder it takes forever.

Example: I receive a lot of files over the internet. When I click “Save As” the window opens immediately - but is empty for at least 15 to 20 seconds, sometimes longer, before it shows the files in the folder. Same thing happens when I try to upload files.

Strange thing is: When the folder is already open in Windows Explorer there is no problem at all: I can open files, move them to a different folder or even delete them.

The NAS software is current and no errors are shown. Disabling sleep mode didn’t help either.

As this behavior is slowing down my work considerably: Does anybody have a clue on “repairing” this issue?

Thanks in advance.

Partly solved.
Obviously this was a Windows 10 bug, partly solved by some update (?). Now when the Explorer window for the NAS opens, it is no longer empty. But clicking “Save As” or “Open” still takes a while until something really happens.

So this obviously had nothing to do with MyCloudMirror.