Why does my preloader vanish before the page loads?

My preloader disappears before the content is fully loaded,so technically it's useless. How can I fix it also that I've tried various approaches without success? Also, the gif is incredibly enormous; how can I regulate and reduce the size of the gif? Thank you very much for all of your help.

#loader{
    position: fixed;
    width: 100%;
    height: 100vh;
    background: #ffffff url('https://media.giphy.com/media/xTkcEQACH24SMPxIQg/giphy.gif') no-repeat center;
    z-index: 999;
}

<!DOCTYPE html>
<html lang="en">
<head>
  <link rel="shortcut icon" href="./images/favicon.ico" type="image/vnd.microsoft.icon" />
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>    
    <link href="https://cdn.jsdelivr.net/npm/bootstrap@5.1.3/dist/css/bootstrap.min.css" rel="stylesheet" integrity="sha384-1BmE4kWBq78iYhFldvKuhfTAU6auU8tT94WrHftjDbrCEXSU1oBoqyl2QvZ6jIW3" crossorigin="anonymous">

    
    <!-- Favicon icon -->
</head>
<body> 
     <!-- Preloader -->
     <div id="loader"></div>
     <div id="content">
        
        <img src="https://images.pexels.com/photos/3791466/pexels-photo-3791466.jpeg"></img>
<img src="https://images.pexels.com/photos/3791466/pexels-photo-3791466.jpeg"></img>
<img src="https://images.pexels.com/photos/3791466/pexels-photo-3791466.jpeg"></img>
<img src="https://images.pexels.com/photos/3791466/pexels-photo-3791466.jpeg"></img>
        </div>


       </div> src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.6.0/jquery.min.js"></script>
      <script src="https://cdn.jsdelivr.net/npm/bootstrap@5.1.3/dist/js/bootstrap.bundle.min.js" integrity="sha384-ka7Sk0Gln4gmtz2MlQnikT1wXgYsOg+OMhuP+IlRH9sENBO0LRn5q+8nbTov4+1p" crossorigin="anonymous"></script>
     <script src="./js/script.js"></script>
           <script>var loader;function loadNow(e){e<=0?displayContent():(loader.style.opacity=e,window.setTimeout(function(){loadNow(e-.05)},100))}function displayContent(){loader.style.display="none",document.getElementById("content").style.display="block"}document.addEventListener("DOMContentLoaded",function(){loader=document.getElementById("loader"),loadNow(1)});</script>

Solution

#loader{
    position: fixed;
    width: 100%;
    height: 100vh;
    background: #ffffff url('https://media.giphy.com/media/xTkcEQACH24SMPxIQg/giphy.gif') no-repeat center;
    z-index: 999;
}

<!DOCTYPE html>
<html lang="en">
<head>
  <link rel="shortcut icon" href="./images/favicon.ico" type="image/vnd.microsoft.icon" />
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>    
    <link href="https://cdn.jsdelivr.net/npm/bootstrap@5.1.3/dist/css/bootstrap.min.css" rel="stylesheet" integrity="sha384-1BmE4kWBq78iYhFldvKuhfTAU6auU8tT94WrHftjDbrCEXSU1oBoqyl2QvZ6jIW3" crossorigin="anonymous">

    
    <!-- Favicon icon -->
</head>
<body> 
     <!-- Preloader -->
     <div id="loader"></div>
     <div id="content">
        
        <img src="https://images.pexels.com/photos/3791466/pexels-photo-3791466.jpeg"></img>
<img src="https://images.pexels.com/photos/3791466/pexels-photo-3791466.jpeg"></img>
<img src="https://images.pexels.com/photos/3791466/pexels-photo-3791466.jpeg"></img>
<img src="https://images.pexels.com/photos/3791466/pexels-photo-3791466.jpeg"></img>
        </div>


       </div> src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.6.0/jquery.min.js"></script>
      <script src="https://cdn.jsdelivr.net/npm/bootstrap@5.1.3/dist/js/bootstrap.bundle.min.js" integrity="sha384-ka7Sk0Gln4gmtz2MlQnikT1wXgYsOg+OMhuP+IlRH9sENBO0LRn5q+8nbTov4+1p" crossorigin="anonymous"></script>
     <script src="./js/script.js"></script>
           <script>var loader;function loadNow(e){e<=0?displayContent():(loader.style.opacity=e,window.setTimeout(function(){loadNow(e-.05)},100))}function displayContent(){loader.style.display="none",document.getElementById("content").style.display="block"}window.addEventListener("load",function(){loader=document.getElementById("loader"),loadNow(1)});</script>

TLDR; Changed document.addEventListener("DOMContentLoaded" to window.addEventListener("load" because of their different behavior