WebCamTexture and HTC Vive - lagging

I am using a Magewell USB Capture device to convert a HDMI input signal from an other computer into a WebCamTexture on a virtual screen. When using Oculus this works like a charm, but when using HTC Vive (OpenVR) the picture lags a lot when rotating the head.

Does anyone know why and how I can fix this?

This might have to do with the frame rate of the captured video from the Magewell device (XI100DUSB-HDMI Video)

If I set the WebCamTexture.requestedFPS=30 the lagging seems to go away. At least on my desktop computer…