React Hook recipe from https://usehooks.com. Demo: https://codesandbox.io/s/jjxypyk86w

use-memo-example.jsx

import { useState, useMemo } from 'react';

// Usage
function App() {
  // State for our counter
  const [count, setCount] = useState(0);
  // State to keep track of current word in array we want to show
  const [wordIndex, setWordIndex] = useState(0);

  // Words we can flip through and view letter count
  const words = ['hey', 'this', 'is', 'cool'];
  const word = words[wordIndex];

  // Returns number of letters in a word
  // We make it slow by including a large and completely unnecessary loop
  const computeLetterCount = word => {
    let i = 0;
    while (i < 1000000000) i++;
    return word.length;
  };

  // Memoize computeLetterCount so it uses cached return value if input array ...
  // ... values are the same as last time the function was run.
  const letterCount = useMemo(() => computeLetterCount(word), [word]);

  // This would result in lag when incrementing the counter because ...
  // ... we'd have to wait for expensive function when re-rendering.
  //const letterCount = computeLetterCount(word);

  return (
    <div style={{ padding: '15px' }}>
      <h2>Compute number of letters (slow 🐌)</h2>
      <p>"{word}" has {letterCount} letters</p>
      <button
        onClick={() => {
          const next = wordIndex + 1 === words.length ? 0 : wordIndex + 1;
          setWordIndex(next);
        }}
      >
        Next word
      </button>

      <h2>Increment a counter (fast ⚡️)</h2>
      <p>Counter: {count}</p>
      <button onClick={() => setCount(count + 1)}>Increment</button>
    </div>
  );
}

FYI looks like this example errors in codesandbox due to infinite loop

Yes a big infinite loop is the problem

@danielsdesk @vinayakshahdeo Thanks for letting me know. It wasn't actually an infinite loop, but Codesandbox didn't like the intentional 1,000,000,000 iterations in the computeLetterCount function. I was able to disable infinite loop protection so that the Codesandbox runs now :)