I´m using the GTX 1080 with Displayport->Displayport to Monitor. The overall performance on the system is great, there is NO problem with anything.
Except using the UE4.11/.12/.13 (not testet with earlier versions). UE4 runs nice using displayport as long as the viewport has the focus (120fps), but it lags as hell when the viewport loses the focus (4-6fps). Opening another window (Blueprint editor) crushes the performance even if the viewport has the focus. Minimizing the other window (Blueprint editor) will give the performance back for the viewport.
Using Displayport->HDMI (Adapter) with HDMI-HDMI (cable) works nice.
Using HDMI-HDMI (cable) works nice.
Using HDMI-HDMI, starting UE4, switching to Displayport->Displayport works nice.
Only if Displayport->Displayport is used when UE4 is started we have this issue.
And yes, I tried other ports on the card, and the newest driver for 1080 is installed.
Displayport->Minidisplayport for HTC Vive is not working either. Not Epics fault, but may help with the issue.
Monitor is 4k, downscaling or changing resolution does not help either.