Memoized LetterInput

Form.js

const MLetterInput = memo(
  LetterInput,
  ({ value: _old }, { value: _new }) => _old === _new
)

//... Inside the render method of <Form/>

      {Object.keys(alphabet).map((l, i) =>
        state.itMatters ? (
          <MLetterInput
            key={l}
            l={l}
            i={i}
            value={state[l]}
            onChange={changeLetter(l)}
          />
        ) : (
          <LetterInput
            key={l}
            l={l}
            i={i}
            value={state[l]}
            onChange={changeLetter(l)}
          />
        )
      )}