Just a thought, but I wonder whether the slow speed on the USB-C connection is being caused by the fact that you are running the Production release of Roon on your Titan?
There is an Early Access release of Roon and RoonOS that apparently fixes some issues with audio over HDMI and support for USB-C Thunderbolt.
It should be rolled into a future Production release in the not-too-distant future, if you can wait.