Due to the high resolution of the hires camera (4056x3040), the voxl-portal has a hard time encoding these images to jpegs (using a software jpeg encoder turbojpeg), so that they can be sent out to your browser. This is expected behavior. In order to avoid this, please choose a lower resolution stream, something closer to 1280x720, or what you already have as an option 1024x768 hires_small_color (instead of hires_large_color).
Voxl-portal is a tool that should be used for debugging / testing and is not optimized for efficient video transmission.
For most efficient video transmission, you should use the encoded stream and you could make it available via rtsp protocol using voxl-streamer tool. Then camera server will use hw video encoder to generate the video, send it to voxl-streamer and the streamer can forward the video to you rtsp player (vlc, ffmpeg, mplayer, etc, etc)
If you need more guidance, please let us know what you are trying to achieve.
Alex