Imagine you tap a button on your banking app to transfer money, and it takes 30 seconds to respond – or worse, the whole app crashes. Today, so many things depend on software and the internet that a slow or broken service isn’t just annoying; it can lose customers, damage trust, and cost real money. This chapter is about making sure that doesn’t happen. We’ll explore how to measure what “good performance” actually means and how to build systems that can bend without breaking, keeping things fast, available, and reliable even when stuff goes wrong.