Apple’s guidelines have always said that developers should avoid splash screens ‘or other startup experience.’ That's nothing new, but many developers ignore it anyway.
Easier said than done. This is trivial for single-purpose apps like Calculator or Weather, where it's easy to predict what the post-launch UI will look like (hint: there's only one screen...) For much larger apps the "fake screenshot" is harmful. Take the Facebook app for example - what should the "fake screenshot" be? iOS does not differentiate between a completely fresh launch vs. a simple restore from background,…
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As for slow app startups, Apple hates that too. It's fine to show a loading screen, if there is loading to be done. Showing marketing videos and other bullshit? No. Not fine.