Using this example (same customizations of amazon-cognito-identity-js library) : https://github.com/torrens/react-native-aws-cognito-expo
I have some performance issues. In fact, we cannot use directly amazon-cognito-identity-js because it needs to be linked.
Amazon has been some enhancements (see enhance-rn.js) for react native supports and all these enhancements concern BigInteger. If Expo could propose on BigInteger implementation, It would be very easy to have a performant amazon-cognito client (I could make one).