React: How to prevent a setInterval from being delayed after a button is clicked to start it?

In this React Pomodoro Clock, there is a function countDown which starts setInterval, however, when the button id="start_stop" is clicked to start countDown, there is some delay before starting. Any help would be greatly appreciated. CodePen.io: https://codesandbox.io/s/tender-wozniak-kfuss

index.js

import React from 'react';
import ReactDOM from 'react-dom';
import './style.css';

/*
* A simple React component
*/
const initState = {
  breakLength: 5,
  sessionLength: 25,
  init: 'session',
  stateIndex: 0,
  timeLeft: undefined,
  timeLeftSeconds: undefined,
  started: false,
  intervalFunc: undefined
}

const states = [ { name: 'session', duration: 1500 }, { name: 'break', duration: 300 } ]

const secondsToMins = (time) => {
  let converted = ('0' + Math.floor(time / 60)).slice(-2) + ':' + ('0' + Math.floor(time % 60)).slice(-2);
  return converted;
}

class Clock extends React.Component {

  constructor(props) {
    super(props);
    this.state = initState;
    this.breakDecrement = this.breakDecrement.bind(this);
    this.breakIncrement = this.breakIncrement.bind(this);
    this.sessionDecrement = this.sessionDecrement.bind(this);
    this.sessionIncrement = this.sessionIncrement.bind(this);
    this.startStop = this.startStop.bind(this);
    this.reset = this.reset.bind(this);
  }

  componentDidMount() {
    let sessionSeconds = this.state.sessionLength * 60;
    this.setState({ timeLeftSeconds: sessionSeconds });
    this.setState({ timeLeft: secondsToMins(sessionSeconds) });
  }

  breakDecrement() {
    // decrements the breakLength and the breakSeconds
    // breakLength is only a number ie. 5 (does not show seconds)
    // breakSeconds is that nunber converted into seconds
    let breakLength = this.state.breakLength - 1;
    if (breakLength > 0 && breakLength < 61){
      this.setState({ breakLength: breakLength });
      let breakSeconds = breakLength * 60;
      states[1]['duration'] = breakSeconds;
    }
  }

  breakIncrement() {
    // same as decrement except does increment
    let breakLength = this.state.breakLength + 1;
    if (breakLength > 0 && breakLength < 61){
      this.setState({ breakLength: breakLength });
      let breakSeconds = breakLength * 60;
      states[1]['duration'] = breakSeconds;
    }
  }

  sessionDecrement() {
    // decrements the sessionLength and the sessionSeconds
    // sessionLength is only a number ie. 25 (does not show seconds)
    // sessionSeconds is that nunber converted into seconds
    let sessionLength = this.state.sessionLength - 1;
    if (sessionLength > 0 && sessionLength < 61){
      states[0]['duration'] = sessionLength*60;
      this.setState(prevState => ({
        sessionLength: prevState.sessionLength-1,
        timeLeftSeconds: (prevState.sessionLength-1)*60,
        timeLeft:  secondsToMins((prevState.sessionLength-1)*60)})
      );
    }
  }

  sessionIncrement() {
    // same as decrement except does increment
    let sessionLength = this.state.sessionLength + 1;
    if (sessionLength > 0 && sessionLength < 61){
      states[0]['duration'] = sessionLength*60;
      this.setState(prevState => ({
        sessionLength: prevState.sessionLength+1,
        timeLeftSeconds: (prevState.sessionLength+1)*60,
        timeLeft:  secondsToMins((prevState.sessionLength+1)*60)})
      );
    }
  }

  startStop(id) {
    // starts the countDown, which runs continuously until the start/stop button
    // is pressed again, which pauses the countdown.
    // the id parameter is used by countDown to play the audio beep
    if(!this.state.started){
      this.countDown(id);
      this.setState({ started: true});
    }
    // pauses the countDown
    if(this.state.started){
      let intervalFunc = this.state.intervalFunc;
      clearInterval(intervalFunc);
      this.setState({ started: false});
    }
  }

  reset() {
    let intervalFunc = this.state.intervalFunc;
    clearInterval(intervalFunc);
    // reset state to default values
    this.setState({ breakLength: 5 });
    this.setState({ sessionLength: 25 });
    this.setState({ init: 'session' });
    this.setState({ timeLeftSeconds: 1500})
    this.setState({ timeLeft: '25:00' });
    this.setState({ stateIndex: 0 });
    this.setState({ started: false });
    this.setState({ intervalFunc: undefined });
  }

  countDown(id){
    // set the function to a variable and set state to it, so the function
    // can be paused with clearInterval()
    var intervalFunc = setInterval(() => down(this.state.timeLeftSeconds--), 1000);
    this.setState({intervalFunc: intervalFunc});

    const down = (time) =>
    {

      if(time > 0){
        // converts seconds to MM:SS at every t-minus
        this.setState({ timeLeft: secondsToMins(time)});
        console.log(time);
        console.log(this.state.timeLeft);
      }

      if (time <= 0) {

        let sound = document.getElementById(id).childNodes[0];
        sound.play();

        let stateIndex = (this.state.stateIndex + 1) % states.length;
        this.setState({ stateIndex: stateIndex});
        this.setState({ timeLeftSeconds: states[stateIndex].duration});
        this.setState({ init: states[stateIndex].name});
        this.setState({ timeLeft: secondsToMins(time)});

        console.log(time);
        console.log(this.state.timeLeft);
        console.log(this.state.init);
      }
    }
  }

  render() {
    return (
      <div id="clock">
      <h1 id="title">25-5 Clock</h1>

      <div>
      <p id="break-label">Break Length</p>
      <p id="break-length">{this.state.breakLength}</p>
      <button id="break-decrement" onClick={e => this.breakDecrement()}> Decrease </button>
      <button id="break-increment" onClick={e => this.breakIncrement()}> Increase </button>
      </div>

      <div>
      <p id="session-label">Session Length</p>
      <p id="session-length">{this.state.sessionLength}</p>
      <button id="session-decrement" onClick={e => this.sessionDecrement()}> Decrease </button>
      <button id="session-increment" onClick={e => this.sessionIncrement()}> Increase </button>
      </div>

      <hr/>

      <div>
      <p id="timer-label">{this.state.init}</p>
      <p id="time-left">{this.state.timeLeft}</p>
      <button id="start_stop" onClick={e => this.startStop(e.target.id)}><audio id="beep" src='./beep.mp3'></audio> start/stop </button>
      <button id="reset" onClick={e => this.reset()}> reset </button>
      </div>

      </div>
    );
  }
};

/*
* Render the above component into the div#app
*/
ReactDOM.render(<Clock />, document.getElementById("app"));

index.html

<!DOCTYPE html>
<html lang="en" dir="ltr">
<head>
  <meta charset="utf-8">
  <title>25-5 Clock</title>
  <style>
  </style>
</head>
<body>
  <main>
    <div id="app"></app>
    </main>
 </script>
  </body>
  </html>