I'm developing a videogame with QGraphicsScene and some QGraphicsItem in my Nokia device. I use to have a big scene (3000x1000 pixels sometimes) with the viewport following the main character trough it.
I'm having two performance problems:

1) I use to have around 50-150 QGraphicsPixmapItems in my scenes, and I'm trying to improve the performance caching them using "ItemCoordinateCache". It goes fine in all the scenes BUT in an specific place in one of them. I can solve it not caching those items but I would like to know what can be happening.
I guess there are many of them (20) at the same time on the screen.

2) Which options do I have to display a background image? so far I've tried
a) As I can't have so big QPixmaps, I divide the jpg file in pieces and I create a QGraphicsPixmapItem with each one.
b) As "point a", but painting them in "drawBackground" method.

Should I try to make the cache bigger? will it have problems if I upload it in OviStore? will I be able to use the big QPixmap if I do it? which size should I try?
Right now I'm using the default size.