I'd see if the video drivers and/or the SOC drivers need to be updated... also, try a refresh or even restore of Win 8/8.1 if necessary (though you'll have to reinstall non-modern apps and files which can be a pain).
Is this the XPS 12 with the rotating screen? Maybe there was a configuration with USB 3.0 I wasn't aware of... if so, there are some decent USB 3.0 video docks out there in the $50-$100 range (the $15 VGA adapters might require more processor headroom than you have available).