> I think that's sufficiently fast. (Keep in mind that a 4k screen is about 8.5mp.) How fast do you want your slideshow to be?

A modern iPhone can capture at up to 48MP. If the performance scales linearly with pixel count, that would put tapping on a thumbnail to the full size being ready at over half a second. That's going to feel laggy. Now you can throw storage at the problem and pre-compute a downscaled intermediate, sure, but that doesn't fix it when you send the photo to someone else or whatever.

And competitive phones are doing 200mp captures (which is stupid in its own right but phone manufacturers and doing stupid things, name a more iconic duo)