What do you actually have attached to your Thunderbolt/USB-C connector? If it's a TB16, do you have the version with the 240W AC adapter? That's required in order for the TB16 to provide 130W to the attached system, and the XPS 15 requires 130W to operate properly. If it's connected to an undersized power source, it will first throttle performance, and under very heavy loads and/or a severely undersized power source it WILL also pull from the battery to make up the power deficit. You seem to want it to drain from the battery first rather than throttling. I believe (but don't know for certain) that the reason Dell doesn't do that is because they figure that if you have a power source connected, you probably want to charge your battery or at the very least keep it from draining. If the system prioritized performance above all else, then somebody who plugged their system into a power source in the middle of a heavy workload might end up not being able to finish that workload because the battery would die in the middle of the task. Most people would rather have their work slowed down but still be able to work rather than having their system die on them.