So the bit that I obviously failed to make clear was that I'm not trying to produce thumbnails or previews

Yes you didn't - and that changes the problem entirely. There are a number of pixel-interpolating algorithms, and the fastest is "nearest neighbor", or Imager::preview in your terms. A good visual balance for image downscaling is usually produced by the bicubic algorithm, but Image::Magick has also qw(Quadratic Triangle Hermite Hanning Hamming Blackman Gaussian Catrom Mitchell Lanczos Bessel Sinc) (at least those I know), and you might want to try which works best for you.

In reply to Re^3: Resizing large TIF files by dk
in thread Resizing large TIF files by grantm

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