When saving/loading QPixmap to/from QDataStream, it's converted to PNG which takes some time.

Instead, save/load the image bits directly (in a compressed form though). This doesn't need the PNG conversions and is thus quite a bit faster (about 260 vs 980us for reading and 1900 vs 3100us for writing a 48x48 image). And it seems to use ~10% less space as well.