Synopsis: Experiencing extremely low network speed with laptop connected to the monitor's USB Type-C and a GigE USB-3 dongle attached to one of the USB-3.0 ports of the monitor.  Internet speed = ~30 Mbps!  Direct attachment of GigE dongle to laptop yields ~350 Mbps.  

Detailed configuration:

• Laptop: experimented with Lenovo X1 Carbon 6th generation and Microsoft Surface Book 2.
• OS: Windows 10 Pro for both experiments.
• Laptop connected exclusively through the USB Type-C cable to the U3818DW monitor.  No other cables are attached to the laptop.
• Plugable USB-3.0 GigE adapter connected to one of the USB-3.0 ports on the bottom of monitor.  Experimented with both bottom ports and got the same result.
• Experimented with a different brand of GigE adapter: Cable Matters USB-3.0 GigE Adapter.  Identical results.
• In all experiments described above, the Ethernet adapter status in Windows-10 reports the adapter operating at 1GBps.  There are no driver issues with either of these common adapters.

Folks, has anybody seen a similar phenomenon?  I wonder if my monitor's built-in USB hub is broken.

I understand that the USB rate will drop to the USB-2.0 speed once the laptop communicates with the monitor at full resolution.  However, USB-2.0 speed is rated at 480 Mbps.  It should have not created a bottleneck for my Internet speed which cannot exceed 350 Mbps.  At least not this dramatic throttle - 30 Mbps.