HTML

slider is flashing when change background image

<!-- DO A PRELOAD OF YOUR SLIDER'S IMG FISRT-->
<!-- SO PUT DISPLAY NONE IN A RANDOM DIV-->

<div class="slider"></div>

<div class="hidden" style="display:none">
  <img src="https://via.placeholder.com/399">
  <img src="https://via.placeholder.com/400">
  <img src="https://via.placeholder.com/401">
  <img src="https://via.placeholder.com/402">
  <img src="https://via.placeholder.com/403">
  <img src="https://via.placeholder.com/404">
</div>