Quicksave | Quicksave images in Discord | Chat library

Im new to React-Native and currently building a chat-function into my app. To visualize the chat Im using a FlatList (before I used Scrollview but with FlatList I have better performance with multiple items), netherless I have two big problems.

1. As I write more chatmessages more and more messages are getting appended to my chatmessages hook. One message is not a problem, but after 2 or 3 the whole FlatList starts to blink and reload. This gets worse if I append even more messages.
2. When I send more then 3 Messages to the Database, and they get downloaded by the firestore listener to my FlatList, those items arent even properly rendered into my FlatList. I use concat to add them at the end of my chatmessages state but FlatList displays them at the beginning of the whole chat! After that it blinks, and the new message drops one item lower, it blinks again and it drops lower, and this happens so long until the new message finally dropped to the end of the whole chat where it should be. It doesnt make sense at all because I use concat, so the new chatmessages are at the end of my whole chatmessages array, why is FlatList(and also Scrollview) adding them at the beginning and rerendering as much as they need to drop it to the bottom??

My Code looks like this:

The problem I seem to have hit is one relating to loading times; I'm not running on a particularly fast machine by any means, but I still want to dabble into neural networks. In short, I have to load 336,600,000 integers into one large array (I'm using the MNIST database; each image is 28x28, which amounts to 748 pixels per image, times 45,000 images). It works fine, and surprisingly I don't run out of RAM, but... it takes 4 and a half hours, just to get the data into an array. I can supply the rest of the code if you want me to, but here's the function that runs through the file.