Optimization in Graphics
The JPEGutils used QImage::smoothScale to scale JPEGs. Remove this call, so now all scaling is done at one place.
Scale down only if the image is considerably larger (~ >150%).
Scaling with FastScale seems 5x faster (callgrind) than the scaling with smoothScale (takes 4% vs. 20% of the preload time). To me it appears better to use the faster algorithm, even with faint quality loss.
Note: this is not about scaling when drawing, but about scaling when loading the preview.