Using lower resolution/size images for thumbnails on posts index

You will have to edit your theme files for that, but you can use the_post_thumbnail (Codex) to show the featured image in a smaller size.

This is an example from the Codex which you can probably use 1 to 1:

<?php if ( has_post_thumbnail() ) : ?>
   <a href="https://wordpress.stackexchange.com/questions/189741/<?php the_permalink(); ?>" title="<?php the_title_attribute(); ?>">
      <?php the_post_thumbnail(); ?>
   </a>
<?php endif; ?>

It works fine and helps a lot with getting better load times.