External SSD drive slow on USB3 on MacBook Pro

Hi, I hope someone can help me before I send this product back.

I have just purchased the WD MY PASSPORT SSD 256GB for my MacBook Pro (Retina, 13-inch, Early 2015), which has USB3.

The transfer speeds should be around the 500MB/s mark. I wanted the fast read/ write speeds for video editing on Premiere Pro.

I have checked the speed using Black Magic Disk Speed Test and I can only get around the 200MB/s which is not what I was expecting at all.

Is there something I am missing here? Any advise would be appreciated as I don’t want to have to send the drive back.

Thanks in advance,

Callum.