Correction - You said PCIe x4 v3 which has a max bandwidth of 3940MB/s

The external enclosure is rated to max out at the same speed as the Dell. Why would it only use a fraction of the speed that is available, even with a bottleneck it should at least reach the bottleneck. 

Are you using the USB/C port on the back of the PC.  That USB/C port is twice as fast as the one on the front of the PC.  

Correct. I am getting identical speeds internal via pcie AND via external enclosure. Approx 225/175MB/s on the R10. 

Significantly slower than the enclosure (and pcie card, for that matter) @ 1000/1000 it is rated for, and achieves on my Intel MBP. 

Even if there were some devices or peripherals (there aren’t), and even if we assume the port splits to 2x5gbps that’s still >600MB/s. 

The bus lanes for USB are likely shared and could also be shared with other components, like PCI lanes. So if the total lane for USB has a max of 10Gbs and you have something plugged into a 10Gbs port and something plugged into a 5Gbs port, your 10Gbs device is actually only going to run at 5Gbs because it has to share the lane.

I have no idea how the R10's lanes are configured (It's not published) so you'd need a Dev or support to confirm that here.

What expansion card did you use? 

Ugh, I'm getting Gigabits and Gigabytes confused again. I really wish that were standardized. So that 10GBs rating on the USB port is actually 10 Gigabits which equates to 1.25 Gigabytes. Half of that would be the 625 Mbs you're describing. That being said I'll shut up and let someone else answer who isn't having a brain fart.

Macbook uses Thunderbolt (40Gbs) for the connection, which is always going to be faster than any port on most desktops. By contrast, the fastest port on the R10 is a single USB 3.1 Gen 2 port (10Gbs)

And the pcie expansion is running at the same speed as external enclosure. 

How is that accounted for?