It sounds like the system likely has a discrete GPU -- the reason for the 130W adapter.  It will not run at full speed on a 90W power supply from the monitor. The system will run from only one input at a time, and if you want it to run at full performance, you must use the OEM Dell adapter. The other alternative is a Dell docking station capable of supplying 130W to the system (which itself will have a 180W or larger capacity power supply).  
Have XPS 15 9520 that is predominately used at a desk connected via USB C to an external monitor (LG 34WP85C) Supplied power brick is 130W LG Monitor provides 95W via USB C I have set up a Custom Power profile so that the battery charges to a maximum 80% and only starts charging again when it depletes to 50%. This is to prolong the lifespan of the battery. However, when connected to the monitor only via USB C within this charge window it get a message that performance may be affected. Battery charge remains constant. How do I over come this situation? I want to prolong the lifespan of the battery but want best performance. Do I connect my power adapter as well as the monitor or is the answer with on of the other power settings like Primary AC Use within Dells Power Manager. Seems a shame to have two connections. Thanks in advance.
XPS 9520 USB-C Power, Battery Care & Performance