Hi,
This looks expected. It is similar to using nv3dsink or nveglglessink in gstreamer. It takes certain GPU usage. If you would like to eliminate the loading, please use nvdrmvideosink in gstreamer or use NvDrmRenderer in jetson_multimedia_api.
For interested people, I was able to reduce the GPU usage of my Qt application that shows 2 video stream by one global repaint instead of 2 local repaint.