It's just the amount of things each app loads. Amethyst loads a TON of information when you visit each profile. It's all goes in memory. So, larger profiles will take more resources from the phone. Lots to optimize yet.
I'm not a dev, but I suspect the differences may have to do with Amethyst being a mobile app, and Iris being web-based.
Tagging nostr:npub1gcxzte5zlkncx26j68ez60fzkvtkm9e0vrwdcvsjakxf9mu9qewqlfnj5z so he can see this feedback and chime in if he is so inclined...
Discussion
You are doing an awesome job, btw! 👍
If that is the only difference, perhaps the strategy might be to load only the minimum, until some user action requires more???😳😮
Many strategies are possible. The problem is time to work on it :)