WD15 Slow TFTP WIM transfer

We are in the process of trialing some new mobile devices for our fleet and having an issue with PXE booting them with the WD15 dock.

The models specifically are:

* Latitude 5175

* Latitude 7275

* Latitude 7370

Using SCCM 1602, WinPE10 boot images.

PXE Boot works but the transfer of the initial WinPE WIM is so slow! Takes about 3-4 minutes just to transfer the WIM image (about 250MB). Once past the initial transfer speed is normal for the rest of the OSD deployment.

Things we have done:

* Added all relevant drivers

* Upgraded all firmware to latest version

* Enabled 'Thunderbolt' networking in BIOS

* Disable Thunderbolt security

* Change POST to 'Thorough'

Moving the 5175 to the older style dock (K10A) and the speed is back to normal. It seems to be directly related to the Thunderbolt TypeC dock. A normal PXE boot from any other dell device / dock takes around 20 seconds at most.

Is there something I'm missing here?

I've seen mentioned to adjust the 'RamDiskTFTPBlockSize' but we are not able to as it breaks our VMWare deployments. I don't think that is the answer anyway as that applies to general speed problems not specific device problems.