When displaying a significant number of records, it is often a basic functions in a web application is to paginate the records and load them as requested.
Create Acceptance Tests or Feature Tests to extend your automated tests with browser testing. It can help tests some areas that cannot be reached with other types of tests.
Using ActiveModel::Serializer, learn how to create the JSON responses for your Rails API application. ASM is a great object oriented approach to building a JSON API.
Add multi factor authentication to your existing authentication solution. This gives your users the option to increase the level of security to their account and help prevent unauthorized access.