I don't use andy dongle/converter. The 2d displays shouldn't use much power at all, but maybe it will help to plug one of them to the built in graphics? The main thing though is that my computers CPU and GPU don't even work near its full capacity. Still lagging.
I use 90 hz refresh rate, that might be a bit much. But the VR headset is still only 14402880 = 4 Mpixel. It should not be too hard to handle. Even if a video is higher resolution the displays on the headset will still have 14402880 pixels to render. I cannot find a setting in SteamVR to lower the refresh rate.