qgis - Why does intersect not calculate polygon areas?

this is strange: I intersected 2 shape-files and the resulting shape does not contain the Shape_Area and Shape_Length of the intersected polygons. Instead the field Shape_Area contains the polygon-area of one of the 2 original Layers.

Recalc of the geometries with vector->geometry->add/export column seems to add the areas. That is quite a drag and very slow for my 350.000 polygons. So I am hoping for a systematic approach that adds the areas while generating the intersect.

Is intersect done by QGIS (qgis.bin) or by a gdal-tool? Either way: is there a "calculate areas"-option hidden somewhere?

THX, stn