When working on Scene graph type of channel, after the initial splash screen and before the actual first screen being displayed there is the black screen..I understand this is the time while all components are being initialized.
Mine is lasting for about 7 to 8 seconds, I have a bunch of different screens (total of 6, with some fading background and an URL handler for each of the screen)...
I have 7 different type of screen...I am managing all screens visibility from the Scene so all screens are defined in Scene.xml.
Well, I have just started to develop channel so I am also not an expert yet, but my understanding is that there should be only one scene, except for screensavers that should be defined in a separate scene...
One thing to consider is that once you create first RSG scene Roku starts process of "compiling" all files in the components directory. Depending o how many components you have it could take a second on faster device or a few seconds on slower devices.