Secondary monitor frame rate problem

[Roocher]

Hey there

I´m using dolphin on a system with two displays.
Display 1: 2560 x1440 desktop monitor - primary
Display 2: Full HD TV - secondary

When running dolphin on my primary desktop display all is fine. Constant frame rate, sound is good.
Changing to my secondary TV display (HDMI) results in fuzzy sound and a poor frame rate.

Is this a known problem?
I tried several display modes (duplicate, expanded). Same problem.
I´m running Win 10 build 17763.379.