shprung

Less than 40 lines of php code so it will not slow your site and if all you 
need is to add an rss feed, give it a try.
You can even setup the output template – something many other plugin don’t 
offer.
<h3>A brief Markdown Example</h3>
<pre><code>short code examples:

[rss url="http://www.usacycling.org/rss/headlines.rss"]


[rss url="http://www.usacycling.org/rss/headlines.rss" no="20" target="_self"]

You can change the template used via wp-admin/options.php , look for
'rss_shortcode_template'
Template fields documentation:
 _TITLE_ : the rss feed title line
 _LINK_ : the link to the complete store, aka permalink
 _MSG_ : the short message/description; the first few 
 lines of the post/story
 _DATE_ : post date & time
 _TARGET_ : the link target
</code></pre>

RSS shortcode w/template

lets you embed rss feed in any WordPress content (e.g., pages, posts, custom post types) using Shortcode.

MailerLite

This plugin automatically adds featured images of your posts into the RSS feed.
Features:
<ul>
<li>Automatically adds featured images of your posts into the RSS feed</li>
<li>Choose from all available image sizes</li>
<li>Choose from multiple image alignment options</li>
</ul>

Featured Image in RSS Feed by MailerLite

This plugin automatically adds featured images of your posts into the RSS feed.

RSS shortcode w/template vs Featured Image in RSS Feed by MailerLite