OTG drives are not slower than other drives, as a class. However, what I have found is that some drives are slow and some are fast. I spent some time benchmarking my own USB drives and found that I have USB 2.0 thumb drives that are faster than one of my USB 3.0 thumb drives; it's just a crummy drive on a USB 2.0 or 3.0 port.
I have two sets of USB 3.0 ports on my machine, one set from the motherboard and one from an add-in card. USB 2.0 drives run about the same speed on both; USB 3.0 drives run significantly faster on the add-in card.
So here's my guess. It's a slow drive. Just because it has a USB 3.0 interface doesn't mean it's fast; it may be just marketing hooey. Remember, all the HDDs these days come with SATA III interfaces even though they couldn't saturate SATA II; I think that's because it's cheaper to have only one kind of interface in stock rather than using one for HDDs and one for SSDs.
I suggest that you time the drive in question on USB 2.0 ports, and on 3.0 ports on another machine. If it's still slow, then it's just a slow drive.