Slowly scrolling auto scroll for testimonials

Hello! I have scrolling testimonials on our website using a Custom CSS code. I have no problem adjusting the seconds between transitions to new slides; however, I'd like to make the transition itself slower, so it feels less jarring. Could someone please provide me with the right code for this?

Let me know! I've pasted my current code below for your reference:
 

<script>
  (function docReady(fn) {
    // see if DOM is already available
    if (document.readyState === "complete" || document.readyState === "interactive") {
      // call on next available tick
      setTimeout(fn, 1);
    } else {
      document.addEventListener("DOMContentLoaded", fn);
    }
  })(() => {
    if ( window.location == window.parent.location ) {
      const reelControls = document.querySelectorAll('section[data-section-id="630305ced17aa33b3d1a4e54"] .user-items-list-carousel__arrow-button--right');
      reelControls.forEach(reelControl => {
        (function reelTimer() {          
          setTimeout(() => {
            reelControl.click();
            reelTimer();
          }, 12000);  // change this value to change the speed
        })();       
      })
    }
  });
</script>
<!-- Google tag (gtag.js) -->
<script async src="https://www.googletagmanager.com/gtag/js?id=G-YQQGCZVL6V"></script>
<script>
  window.dataLayer = window.dataLayer || [];
  function gtag(){dataLayer.push(arguments);}
  gtag('js', new Date());

  gtag('config', 'G-YQQGCZVL6V');
</script>

Have you tried changing number 1200 in the code?

Hi! Yes I did. That lengthened the time of each slide, but not the transition itself. Does that make sense? Is there a way to make the transition slower?