Hey there, refmon!
I'd suggest to test the drive using your HDD manufacturer's diagnostic utility and see what it shows. I'd use several different utilities in your case to make sure everything is fine. If the load is slower, I'd suggest to backup the data from it somewhere else ASAP just to be safe.
Another thing you can do is try the HDD with a different SATA cable and on a different computer to see how it will react there.
If that doesn't help, I'd strongly recommend getting in touch with the manufacturer's tech support for assistance with this.
Hope this helped. Good luck!
SuperSoph_WD