Prioritize Visible Content WordPress

I am running a WordPress site and created a custom theme. Now I’m stuck with optimizing it for Google PageSpeed. It shows the following for the Mobile page:

Consider Fixing: Only about 63% of the final above-the-fold content could be rendered with the full HTML response

This appears since I’ve included the Featured Image above my posts on the index.php with the code:

<p>
  <?php if ( has_post_thumbnail() ) : ?>    
    <a href="<?php the_permalink(); ?>" title="<?php the_title_attribute(); ?>">
      <?php the_post_thumbnail(); ?>
    </a>
  <?php endif; ?>
</p>

How can I fix the issue? It doesn’t seems to me that I’m loading something like a sidebar before the featured image, apart from the logo of the site. For a deeper comprehension, here the complete code of my index.php

<?php get_header(); ?>
  <div id="main">
    <div id="content">
      <?php if (have_posts()) : while (have_posts()) : the_post(); ?>

        <p>
          <?php if ( has_post_thumbnail() ) : ?>    
            <a href="<?php the_permalink(); ?>" title="<?php the_title_attribute(); ?>">
              <?php the_post_thumbnail(); ?>
            </a>
          <?php endif; ?>
        </p>
        <h1>
          <a href="<?php the_permalink() ?>" rel="bookmark" title="Permanent Link to <?php the_title_attribute(); ?>">
            <?php the_title(); ?>
          </a>
        </h1>
        <p>
          <?php the_content(__(' <br /> <br /> Weiterlesen →')); ?>
        </p>
        <hr> 

      <?php endwhile; else: ?>
        <p>
          <?php _e('Sorry, no posts matched your criteria.'); ?>
        </p>
      <?php endif; ?>
      <?php if (show_posts_nav()) : ?>
        <div class="pagination">
          <?php echo paginate_links(); ?>
        </div>
      <?php endif; ?>
    </div>
    <?php get_sidebar(); ?>
  </div>
  <div id="delimiter">
  </div>
<?php get_footer(); ?>

Can someone help me with this issue? What am I doing wrong?