Adam Derewecki wrote down a simple but effective list of attentions you can give your server-side application and to significantly improve performance and scalability. Most of all, they all help to make the most out of the available resources on your (virtual) server and DB.
Coming from a purely iOS background, I know that when building apps for mobile devices it's easy to forget about optimization. At the end of the day, our apps will only run on one machine for one user. However, on the server, every instance of our app runs for potentially thousands of users simultaneously. And our costs for running that application most likely depend on how much CPU and Disk space they use. Therefore, optimizing those things really matters, not only to provide a better user experience, but also to provide a reliable one and to keep our costs down.
Like with the previous link, when I hear about optimization techniques, it often only refers to the back-end, but what about the front-end? Shouldn't we try to optimize that too? If your Swift web application has a somewhat complex website, and it doesn't just focus on serving JSON, you may want to read this. (It's probably and worthwhile read even if you haven't got a website yet. It will help you better understand what happens on sites you use daily.)
In this short article, Madhur Ahuja talks about what he learned over the years about scaling Linux servers. Worth a read, although, most likely, we'll never have to deal with these problems ourselves. This is what DevOps and companies like Amazon and Google do.