Slow write speeds

Just having trouble with my new WD my book 3TB external hard drive. I brought it a month ago and ive noticed that the average transfer speeds for it is around the 15-20 MB/s mark. The files i am transferring are very large (movies and such) the cable and port i am using is USB 3 which should give better transfer speed than this right? Let me know any thoughts/suggestions.
Also i have run the 3 different check up tests available in the smartware but no faults came up

Hi, check if you have the latest USB 3.0 drivers for your computer. You can also try updating the my book firmware.

I rechecked the updates for the hard drive and the ports but it is still slow, is there any ways to speed up the transfers or should i just take it back?

Before returning it you can also do a test with DLG, if the test completes successfully you should try connecting it to another computer.