Slow USB backup on EX4100

I am copying from a WD Black 1TB to my new EX4100 with a jbod 4TB WD Red drive. The old 1TB drive is in a new Voyager S3 USB3 dock attached to the front port of the WD EX4100. I am seeing transfer speeds fluctuating, but all below 3MB/sec. What is causing these slow speeds and what throughput should I expect from this connection?

I’d recommend testing a native USB 3.0 hard drive as opposed to using a dock. If performance improves then you could be experiencing a bottleneck related to compatibility.

Tried My Passport USB3 drive. Same slow speeds uploading.