You may not need that high resolution.
300 dpi is generally for things that are looked at around 12 - 18 inches away - printed A4 documents.
If you're printing out something 20 x 30 inches or more you're probably not intending general viewing closer than a few feet away. In which case you can safely drop to 150 dpi - the viewer isn't going to be able to discern any difference.
Yes, ArtRage does slow down at high resolutions. We're storing and manipulating *much* more data than other digital image applications. As well as colour information we also store paint depth, wetness, shininess, reflectivity.
AndyRage's mantra for graphics engine code:
"Sure - how hard can it be?"