First, you should run tests to check the health of the WD Elements drives using Data Lifeguard Diagnostic tool. Failing this test indicates the error or issue with the drives and need to replace under warranty.
However, poor or slow performance is mostly caused by system configuration issues. It is very rare that the performance issue is directly related to the hard drive. You can refer this article for more details about this.
I will run the lifeguard again and make more tests.
I would like to cite that all tests were performed under the same conditions (pc, system, cables, etc.) and the speed difference is very large between the HDDs.