Here is a photo of the system running right now. Notice the CPU meter on the built-in display averages around 15% of total, but will spike when I run software or do a lot of activity on the display.
When using only one monitor plugged into the built-in Mini DisplayPort output (I use a miniDP-HDMI adaptor), the CPU does not increase with display activity. When a 2nd monitor is added, using the USB connection to the Dell D3100 monitor usb adaptor/dock, the CPU is forced to do work previously performed by the video card. While this consumes 10-25% of the total cpu available (counting all 4 cores as 100%), you will not notice it affecting your daily activity unless you stare at the cpu graph.
The D3100 is able to support more than one monitor, but I have not explored this very deeply (I have briefly tried plugging 2 or 3 monitors into the D3100, with the miniDP port empty). For my full 3-monitor setup at home I have a mid-level ZaReason computer with a regular video card (NVIDIA 750) that works well. It is also on Ubuntu 15.10, although that upgrade was not as smooth as the XPS 13 (the switch to systemd required a change in video card driver install procedure).