i've got some images that are largish (> 1MB) screenshot images, a mixture of vector graphics and gradients, which I've currently encoded as png.

It seems to take prince a few seconds to produce a pdf that has little else besides these.

Is there some way I can prepare an external image file so that prince has to do the least possible
work to embed it?
My understanding is that it passes through jpeg only;
otherwise it converts it to PDF's internal image pixmap format and then applies a standard pdf compression.

I don't know if PDF natively supports JPEG-LS or JPEG-2000, or if those would really be any faster.
My understanding is that both are generally worse than PNG in lossless compression.

At the moment I don't think it's possible to include lossless images without going through the decompression/compression cycle. Are they local files, or accessed over HTTP? A few seconds is a long time, even for large images.