[NAME] The XPS 15 9500 only supports DisplayPort 1.2 over USB-C/TB3.  A TB3 link is capable of carrying two DP 1.2 interfaces, but not all systems and displays support aggregating those in order to drive a single display.  Typically they're used to drive multiple displays over a single cable, as with a Thunderbolt daisy chain or a Thunderbolt docking station.  A single DP 1.2 interface is just barely capable of running 4K 60 Hz, so 3840x1600 75 Hz might be the most you can get from a DP 1.2-based system.  DisplayPort 1.4 supports more bandwidth per interface, which might be how you'd be able to run that display at its native resolution and 144 Hz, but the 9500 doesn't support that.  You'd need the XPS 15 9510 for that.
I have an XPS 9500 and a Thunderbolt wide screen monitor 3840x1600 (LG 38WN95C). I've connected the laptop via the two left Thunderbolt ports directly to the monitor's thunderbolt port (using the supplied USB-C type cable) and the best I can get is 75 Hz. Windows won't recognize anything higher. I've tried using CRU to push it manually and did get it to 97 Hz just to test if the TB3 connection could handle more. Is there anything I'm missing? I have a feeling that something is keeping it stuck at half of 144 but not sure if there is a Dell, Bios, or Intel setting I'm missing.
Can't Get 144hz On Thunderbolt 3 Monitor