I've got a Dell Precision 5510 (skylake) hooked to an external monitor via a Displayport (though same problems came up with HDMI) using two different Displaylink docks. When on Intel graphics, the framerate on the external monitor is appalling – around 1fps when running glxspheres. Using optirun, it's way more (200 fps, same as on laptop monitor). If I switch to nvidia Prime graphics, the laptop monitor shuts off completely, but the external monitor performs very well (2000+ fps).
Using Intel graphics, Chrome became unusable on the second monitor, until I disabled hw accelration in Chrome settings, when it became quite usable. A big problem I'm facing is that the terminal on the external monitor is extremely laggy due to the 1fps problem. Is there a way to overcome this problem, or even disable hardware acceleration entirely; I do have tonnes of cpu to spare.
It appears if I connect the monitor with an HDMI cable directly (instead of through either of the Displaylink docks, then the second monitor gives 60fps using intel graphics. It does mean running another cable, but I guess that's what I'll have to do for now.