THX for all the comments
As a mof I am doing a village - Darkware: how did u know? - the specific problem is that all the material and textures (which are f.e. the same for all windows) from all the other scenes are now 'referenced' to the main scene, too. That means that I have then about 20 window materials in the main scene which are all the same. Now this is really slowing down my O2...
Any more ideas ? - Maybe I'm just blind...