> Is there a way to disable this multiple-pass approach to get a slightly larger image that is generated faster?

Use png_set_filter() with PNG_NO_FILTERS.

> This one makes it look nearly equivalent to zlib

I don't know how squash is set up, but decompression performance depends on which library created the DEFLATE stream (see https://github.com/zlib-ng/zlib-ng/issues/326#issuecomment-4...).

A sibling reply to this one tell you how to tell libpng not to bother using different filters and just always use the null filter, but this will make some input images into _huge_ files (maybe similar size to uncompressed image data) so be warned.

If you had software that you knew spat out images with specific properties you could choose the filter by reference to what you know about the images, e.g. imagine you always produce noise anyway, the null filter is fine, if you produce a horizontal gradient you want the SUB filter, if a vertical gradient, the UP filter. But you'd need to make custom PNG code not rely on a generic library.