Well, on the Series 40 Nokias (like the 7210) the problem is with the non-compacting GC. Loading big images late in the game might be problematic because even when there is enough free memory, it might be too fragmented and there won't be a block big enough for the image. This problem exists in many other phones, but seems to be a lot worse on the Nokias.
to get the performance hole i checked a button deactivating the rendering of the background.
every time, the bg didnt get rendered the performance increased. but the bg-image was still in memory i only switched a flag inside the program.
The background is still in memory, but the thing that is slowing you down is when you try to paint it. If an image doesn't fit in the video RAM, then the actual process of drawing it onto the screen (copying the bits from the regular memory to the screen's buffer) becomes extremely slow. If you're not drawing the image then there shouldn't be any effects on performance, except for maybe the fact that the garbage collector might kick in because you're running low on memory (but this is very easy to control). Just having the image loaded into memory doesn't slow you down.