Anyway, there is a lot of reasons to keep cool
Why not using sprites for characters then? And leave planeA & B for background&foreground?Rendering the 3D characters to layer B is possibly the only time consuming thing. You may have to mask the character against the background, but a pseudo z-buffer could easily be created with each screen. And, depending on the scene, it could be a simple mask/don't mask bit per pixel.
Using sprites would allow more tricks like this : upload top of character if top is moving, upload bottom of character if bottom is moving..
Also, sprite can have palette tricks while backgrounds are more limited...
Another advantage is that sprites can be placed at any X/Y in real time, even if the drawing rate don't fallow...
In other words :
Even if the drawing rate of each sprite (average) is 4fps, the player can feel something quite smooth
Off topic :
In a case of a starwing game, nobody took a look at shadow/stellar-assault 32x and the way the programmers rendered the earth/sun atmosphere? This method seems quite simple, use few colors, and it is very immersive and impressive... (if you havent reached the last level, nevermind... i'm just asking in the case someone saw the last sun level).