Would it be possible for you to get what phone and version of the OS they are attempting to run your app with? Are you doing anything special during the app bootstrapping? without more details it’ll be hard to give you any solid advice.
It should be fast enough. It may be a process that you have happening when the app first loads that’s taking time.
I would try and see if it’s just the Expo framework that’s slow on your device(s) or if it’s a process you have in your app that’s making it slow.
If it’s a process you have running that’s slow try changing your code so you wait until the app has been booted. Maybe have a splash page so the user knows something will happen and make it appear to not be as slow.
I don’t believe you can make the app “faster” from the config.