i just tried a workaround with the trial version of the new keyframe animation plugin. though its intended for object animations it has a tween feature that automatically creates all the transition frames between scenes in a separate file with a specified framrate.
after creating the new file with all the tweened frames i exported the file again as an animation with transition and delay set to zero, which the plugin also does automatically.
sensational result: the animation export to png with imagesize set to full screen resolution took only 15min at 25fps. the creation of the tweens went nearly instant.
this is great, i think i will invest into this nice plugin. only downside, the tweening process ignored hidden objects in some scenes. i will have to see if i can find out why...