React’s sophisticated reconciler (aka the “Virtual DOM”) makes updates to the UI feel fast because it optimizes interactions with the DOM. As a result, we no longer have to worry about how to transition our UI from one state to the next. However, the reconciler is not a silver bullet; even the new Fiber Reconciler. Our React apps can still get sluggish, especially as they grow in size and data. Let’s discuss some causes for slow-downs and tools to diagnose problems so that we can speed up our apps.