One of my external drives (WD My Book) works very slowly when I try to copy/paste files in it or from it. And it happens when I open the folders as well.
It’s an USB 3.0 device and I tried in USB 2.0 and 3.0. And in different computers.
It seems that it depends on the zone of the disc where the files are copied/pasted.
If you have tested different connections with different computer systems, then it is possible this disk drive is showing signs of health issues. I would recommend running a complete drive test using Western Digital Data Lifeguard Diagnostics for Microsoft Windows, and then proceeding to replace the unit if the test is to fail.