What is the exact model of this WD Elements? If it is a USB 2.0 model then 20 MB/s is normal for most systems, while stronger computers will reach around 40 MB/s. If it is USB 3.0, then check your system drivers and the physical port you are using as it should reach around 90 MB/s on most USB 3.0 systems.
That is about the right transfer rate for USB 2. Make sure all of your USB drivers are up to date. Sometimes the USB ports on a desktop work better than those on front. Also don’t connect through a hub.