QML Camera: Loading QQmlApplication has large delay

[prex]

I'm trying to show a video live stream from a USB device in my Qt5.12 QML interface on Ubuntu 18.04. The device is either a Webcam or a HDMI capture device (connected by USB). Following the examples, I added a QML Camera in my QML code. However, loading QQmlApplicationEngine in my main.cpp is now extremely slow (around 5 minutes until the UI starts). As soon as it is running, everything works as expected. I'm running it on a more than powerful workstation.

This is all I'm doing:

    VideoOutput {
        source: camera
        anchors.fill: parent

        Camera {
            id: camera
        }
    }

Does anyone know why this is so slow? Or is there a better solution to show a live stream from an USB device?

[prex]

I was hoping that I can at least show the rest of the GUI using a Loader. This does not solve the initial problem with the delay, of course. But although the ApplicationWindow opens immediately now, nothing is shown inside until Camera is loaded (ApplicationWindow->StackLayout->...->Loader). I don't understand why Camera and Loader are not working as expected. Nobody seems to have this problems. I also checked the GStreamer version, which is 1.0.

    Loader {
        id: loader 
        asynchronous: true
        anchors.fill: parent

        sourceComponent: Item {
            anchors.fill: parent
            Camera {
                id: camera
            }

            VideoOutput {
                source: camera
                anchors.fill: parent
            }
        }
    }