[drblast]

Because it's a one-time cost and you can cheaply throw more CPU power at the problem, or just wait and then you're done.

If you're doing this at such a scale that encoding speed matters (like if you're Instagram) the major concern would likely be cost, which you're going to save in bandwidth every time an image is viewed.

[pmoriarty]

"you can cheaply throw more CPU power at the problem, or just wait and then you're done"

How am I going to "throw more CPU power at the problem" when I'm using a scanner hooked up to my laptop? Should I buy myself a new, more powerful laptop? That doesn't sound cheap.

As for "just" waiting... my time is valuable, and I have better things to do with it than wait... and, unfortunately, the process of scanning and encoding needs a lot of babysitting from me, so I can't just dump a bunch of photos in my scanner and come back when it's all over if I want even half-decent results, not to mention archival quality images.

In this real-world scenario, a quicker encoding speed would save me a lot of valuable time, and there's no getting around it short of hiring a skilled and trusted person to do it for me.

[Nullinker]

Like Tomovo above already said; your workflow would benefit from using an intermediate format that is faster to encode. You can then automate the conversion to the output format to occur unattended at a more convenient time.

The use of intermediate formats is a well proven technique in video editing where encoding real time to AVC/HEVC at high quality is not possible. Codecs like ProRes are used that are much easier to encode to at the expense of storage space.

[tomovo]

Save uncompressed to a big cheap harddrive, batch compress to PNG overnight?