What is "exporting layers" doing that takes so long?

[JasonKleban]

What is "exporting layers" doing that takes so long and how might I make it shorter?

I have 28 layers total, with several of those layers as valuable snapshots reusable in other scenarios.

Until the final image (layers 1-14), I'm running docker with output-type of "cache-only" and then I short-circuit the rest for a fast exit.

The final images (layers 14-28) has the "exporting to image" and "export layers" lines at the end which take 8-12 minutes when those layers weren't yet available from cache. Subsequent similar runs where some of those layers come from cache, the "export layers" is done in 10 seconds.

What is going on here? What should I look out for to avoid causing such a large delay compared to other runs?

Thanks!