I am a newbie at iOS development and I have used NSUserDefaults
to save the username and password of the user.
I now know that I should be using Keychain instead of NSUserDefaults
for this, but I don't have much time and need to publish the app ASAP.
I just want to know if there are chances that my app will get rejected because of this?
This may not be the best thing to do, but it answers your question: Apple won't reject the app.
I have published some apps that use NSUserDefaults to store user credentials. As you told, sometimes it is simply faster.
Just analyse how sensitive your app data should be. Most times the app won't need THAT MUCH security.