It looks like Dell made a small mistake while designing the new XPS 9360, 9550, 9560, and Precision M5510, M5520 laptops. If you are about to use Thunderbolt 3 ports, be aware that you won't reach their full potential as the ports are not running at advertised x4 3.0 (32 Gbps) link.
According to latest information and an XPS 9350 schematic found on the internet, the issue is that the ports running on an x2 3.0 link (16 Gbps), as opposed to the x4 3.0 (32 Gbps) expected of Thunderbolt 3.
This was also confirmed by the GPU enthusiast and developer of egpu.io, Nando Evan (pseudonym), who revealed that the Thunderbolt 3 controller had been hardware-wired for half-bandwidth. Since Dell advertises on their website "up to 40Gbps" speeds with Thunderbolt 3, there could be some lawsuits by customers if the issue is really as it looks like. Stay tuned for more information.