That happens to me too. I suspect it's because as soon as you open Amethyst the loads the posts that are cached on your device then it has to pull the latest notes from all kinds of different relays. It's a slow process.
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The confusing thing is that I didn't even have the app on this phone as far back as some of the posts. That could explain the massive amount of data it uses though.
ya i have weird issues like that too. It's really annoying. Sometimes the notes don't even load and I have to go into the settings and force restart.