Why measure again after each React optimization?
Because optimization is iterative. Some fixes do not help or even hurt, since memoization itself costs something. Measuring again with the Profiler confirms the fix actually improved performance before you keep it.
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More FAQs in A Roadmap to Understanding React Re-renders and Optimization
In order: understand what causes re-renders, use the Profiler, find unnecessary re-renders, stabilize references with useCallback and useMemo, wrap with React.memo, optimize lists, code split, and measure again after each fix.
A component re-renders when its state changes, when its parent re-renders, or when its props change. Understanding this flow is the first step to optimizing, because you cannot fix what you do not understand.
It records a render and shows which components re-rendered and why, including a 'why did this render' feature. This tells you the real bottleneck and the cause, so you optimize the actual problem, not a guess.
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