5C Performance really poor


I have had a 5C for 18 months or so now. The unit was replaced under warranty in June of last year so this unit is 9 months old.
I previous had a iMac and connected to that (via USB C obviously) I would see read and write speeds up to about 200MB/s when measured using the BlackMagic Disk Speed Test
I have recently upgraded to a new Mac Mini. i7, 6 core, 32GB RAM and a 500GB SSD all linked to an external BlackMagic eGPU.
Connected via the same cable directly to the Mac I am now struggling to get 75MB/s write and 130MB/s read on a really good day. I have checked and the device is showing as connected to the Mac as USB3.1.
I tried connecting using a USB-C to USB-C cable and the device would only connect at USB2.0 speeds
Any ideas?
At the moment, my Synology NAS gives me better performance and that’s over a 1G ethernet connection and that can’t be right

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It’s weird, but Drobo suggested that I flip over my USB-C cable. My Drobo was only connecting at USB2 speeds and refusing to connect at USB3 5Gbps. I flipped the connector over, and it was recognized at full speed. Super weird.