Parallax content in a horizontal ScrollMagic container per slide is stuttered

To get the desired effect using ScrollMagic methods you have to set refreshInterval: 1 to the controller2.

Documentantion: http://scrollmagic.io/docs/ScrollMagic.Controller.html#constructor

Example: https://codepen.io/biomagic/pen/dLgzJO

const ease = window.Power4.easeInOut
const el = document.querySelector('#el')
const wrapper = document.querySelector('#wrapper')
const slides = el.querySelectorAll('.El__slide')
const amount = slides.length
const controller = new window.ScrollMagic.Controller()
const horizontalMovement = new window.TimelineMax()

const controller2 = new window.ScrollMagic.Controller({
  vertical: false,
  refreshInterval: 1
})

horizontalMovement
  .add([
    window.TweenMax.to(wrapper, 1, { x: `-${(100 / amount) * (amount - 1)}%` })
  ])

new window.ScrollMagic.Scene({
  triggerElement: el,
  triggerHook: 'onLeave',
  duration: `${amount * 100}%`
})
  .setPin(el)
  .setTween(horizontalMovement)
  .addTo(controller)

slides.forEach((item, index) => {
  const title = item.querySelector('h1')
  const subtitle = item.querySelector('h2')
  const tween = new window.TimelineMax()
  tween
    .fromTo(title, 1, { x: 0 }, { x: 500 }, 0)
    .fromTo(subtitle, 1, { x: 600 }, { x: 500 }, 0)
  new window.ScrollMagic.Scene({
    triggerElement: item,
    triggerHook: 1,
    duration: '100%'
  })
  .setTween(tween)
  .addTo(controller2)
})

Another approach. You can to add tweenChanges: true to your scene.

Documentation: http://scrollmagic.io/docs/animation.GSAP.html#newScrollMagic.Sceneoptions

Example: https://codepen.io/biomagic/pen/rbqpMb

const ease = window.Power4.easeInOut
const el = document.querySelector('#el')
const wrapper = document.querySelector('#wrapper')
const slides = el.querySelectorAll('.El__slide')
const amount = slides.length
const controller = new window.ScrollMagic.Controller()
const horizontalMovement = new window.TimelineMax()

const controller2 = new window.ScrollMagic.Controller({
  vertical: false
})

horizontalMovement
  .add([
    window.TweenMax.to(wrapper, 1, { x: `-${(100 / amount) * (amount - 1)}%` })
  ])

new window.ScrollMagic.Scene({
  triggerElement: el,
  triggerHook: 'onLeave',
  duration: `${amount * 100}%`
})
  .setPin(el)
  .setTween(horizontalMovement)
  .addTo(controller)

slides.forEach((item, index) => {
  const title = item.querySelector('h1')
  const subtitle = item.querySelector('h2')
  const tween = new window.TimelineMax()
  tween
    .fromTo(title, 1, { x: 0 }, { x: 500 }, 0)
    .fromTo(subtitle, 1, { x: 600 }, { x: 500 }, 0)
  new window.ScrollMagic.Scene({
    triggerElement: item,
    triggerHook: 1,
    duration: '100%',
    tweenChanges: true
  })
  .setTween(tween)
  .addTo(controller2)
})