(updated) XPS 9350/9550's Thunderbolt 3 may be hardwired at TB2 speeds
Update: Intel's technology brief for TB3 (p. 5) mention that single-port TB3 SKUs can range from two to four lanes, meaning that Dell's design is technically within specifications.
For the last few weeks, users of Dell's XPS 9350, 9360, and Precision M5510 have been wondering why devices connected to their Thunderbolt 3 (TB3) ports were reported by HWiNFO and CUDA-Z as running on an x2 3.0 link (16Gbps), as opposed to the x4 3.0 (32Gbps) expected of TB3. External GPU enthusiast and developer nando4 (Nando Evan) of egpu.io was able to find the engineering specifications of the XPS 9350 and discovered that the external PCI link TB3 controller had been hardware-wired for half-bandwidth, i.e. TB2-level speeds.
The initial problem was brought to attention by a user inquiring why his AKiTiO TB3 PCIe enclosure with Samsung 960 Pro SSD was under-performing by 50% when connected to his XPS 9350. After confirming via CUDA-Z and HWiNFO that the TB3 controller was running at x2 instead of x4, it was initially thought it was a firmware problem, which Dell could fix with an update. Unfortunately, an XPS 9350 schematic found on the internet shows definitively that the hosting PCIe port is electrically limited to an x2 (16Gbps) link. "No amount of BIOS updates can fix the problem," commented Nando Evan to Notebookcheck in an email.
Why is this a big problem? For one, Dell advertises on their website "up to 40Gbps" speeds with TB3, meaning that if the schematics found are genuine,
Dell could be affected by legal recourse from owners of affected models. The way the XPS 9350 has been shown to be wired will result in speeds similar to TB2, not TB3. As a result, high-bandwidth devices such as Samsung 960 Pro SSDs or external GPUs will be limited to 14xxMiB/s rather than over 2200MiB/s—nowhere near the 40Gbps advertised.
The schematics of the other Dell models affected by lower TB3 speeds (9360, 9550, M5510) have yet to be found—until they are, only the XPS 9350 can be confirmed to have lower speeds due to hardware wiring, specifically. There remains the slight possibility that the other models may only be misconfigured in firmware only, meaning they could be fixed by a BIOS update. However, because of similarities in the engineering, Nando Evan believes it is more likely that all XPS 13/15 chassis models suffer from the hardware limitation.
Disclaimer: This article is based on the information available at the time on writing. Dell has yet to make any official statement on the issue.
Updated 3/1/2017 2:00PM +7 GMT): corrected "TB3 controller" to "external PCI link to TB3 controller", added information on TB3 from Intel clearing Dell of wrongdoing, changed title and abstract.