Six years ago I ran across a framework called Backbone.js and instantly fell in love with the potential of single page applications. Since then I have learned other SPA frameworks and tricks for optimizing performance of an SPA. Today my framework of choice is React and have been coaching others on how to program scalable React applications.
Building a feature rich application is important, but if no one can access the application then the features don't matter. This is why I have made a point of not only learning how to write feature rich applications but ensure they are performant and scalable.