Edit in JSFiddle

let parentRenderCount = 0;
let childRenderCount = 0;

class Parent extends React.Component {
  constructor(props) {
    super(props);
    this.state = {
      unusedField: 1,
      usedField: 1
    }
    setInterval(() => this.setState({
      unusedField: this.state.unusedField + 1,
      usedField: this.state.usedField + 1
    }), 1000);
  }
  
  shouldComponentUpdate(nextProps, nextState) {
    return nextState.usedField !== this.state.usedField;
  }
  
  render() {
    parentRenderCount++;
    return (
      <div>
        Parent {parentRenderCount}
        <div>UsedField: {this.state.usedField}</div>
        <Child />
      </div>
    )
  }
}

class Child extends React.Component {  
  render() {
    childRenderCount++;
    return (
      <div>
        Child {childRenderCount}
      </div>
    )
  }
}

ReactDOM.render(<Parent />, document.querySelector("#app"))