Low transfer speeds over USB-C on Precision 5510

I apologize if this has been covered in one of the previous threads on Thunderbolt 3/USB-C, but I've only recently gotten the USB-C port to work on my machine having set the following three options in my kernel cconfiguration:  

CONFIG_HOTPLUG_PCI_PCIE=y
CONFIG_HOTPLUG_PCI=y
CONFIG_HOTPLUG_PCI_ACPI=y

Together with this dmethod.c patch  I now have USB connections over USB-C (tested with the Ethernet dongle packaged with the laptop, a USB-C to USB-A converter from Google and direct connections via USB-C to a Nexus 6P and Pixel C) working with hotplug support.  However, I have noticed that file transfer speeds to my tablet and phone via MTP over USB-C are extremely slow (a few kbps), much slower than when I connect the same devices to a USB-A port on the laptop with a USB-A to USB-C cable (~20 MBps).  Has anyone else noticed this behavior?  Could I possibly be missing something else in my kernel?  It has been a slow process of trial and error to determine a roughly working configuration but I'm not yet confident that I have everything needed in there yet.

I am still running the original Thunderbolt firmware that came with the laptop as I do not have access to update my firmware, I am not sure if this would affect the USB performance.