React 19 Killed Half My Performance Optimization Code, and I'm Grateful

I maintain a React admin dashboard codebase that had — at last count before upgrading to React 19 — 34 instances of useMemo, 28 instances of useCallback, and 19 components wrapped in memo(). I spent a nontrivial amount of time over two years adding those optimizations, debugging cases where I'd gotten the dependency arrays wrong, and explaining to junior developers why the table re-rendered on every keystroke.

React 19 with the compiler deleted most of that work. Here's what actually changed and what still matters.