Thank you for the link. After reading it, I realize that indeed my images are really too big for a mobile version. On the other hand I cannot use tricks like the re-use of image to create my background because the license does not allow it to me, except a few stones here and there. I have tested a 50% reduction in size of most images and it seems to be working better.
Thanks for your help, i hadn't thought about this problem since the game was loading quite fast.