Hi!
Someone already had this problem?
I was trying change the RSS XML to serve full content of posts in my website, but, when I put the RSS.XML (exactly identical the example of Hugo doc, just changing the .Summary param to .Content), my build dramatically slowed down.
When I remove the rss.xml of _default folder, the build time turn to normal again.
My website is a blog with 1800+ posts. But, usually it build in between 2.5 and 4 secs.

just a opinion: Hugo could serve by default full content at feed.