I think that there is some problem between WD software and USB 3.0 standard explorer copy files from disk inside laptop to external disk. Performance is oscillating from 20 to 40 mb/s. Before installing WD software it was around 90 mb/s on same disks with large files. I have i5-8265u laptop. On desktop and AMD platform everything is OK. Hope that I am not wrong, and that it is not some other problem.
You can try to uninstall the software and install it again to check whether the issue is caused by the software.
Sorry, my laptop died slowly.
So my laptop is back. It was fan so I was thinking not enough cooling. But the problem is still here so I uninstalled complete WD software, restarted. Now without WD software everything is normal. File transfer speed on large files with file explorer from internal disk to usb 3.0 disk is about 100 mb/s stable. It does not oscilate and is not slow. Tested with same files.
I have My Cloud Home connected via wireless. If you first copy something to My Cloud then do file explorer copy from internal disk to usb 3.0 disk then everything is OK. If I see it right. Also after last update of WD software I think that it is somehow better but problem is still here. This is subjective maybe.
Everything is much better now but not in every way. Now on some drives is OK, but on other is not. Speed is better but still it is slowing down. It is manufacturer related, or driver related. On some manufacturers is OK, on some it is not.
Also when there is no internet, and you restart computer without internet WD software is blocking file explorer from working normally. You have to wait a long time from file explorer to respond. Without internet and without My Cloud Home connected. When you connect back to internet everything is normal.