By default the RSS cron task (scheduled task from 3.6 onward) runs every five minutes and checks all valid RSS feeds. This translates into a valid RSS feed being hit every five minutes, which seems excessive. News readers check a few times a day, or on demand. The RDF Site Summary 1.0 Modules: Syndication specification implies that once daily is a reasonable default for feeds which do not publish values for updatePeriod and updateFrequency.

We've had a couple cases of external sites reaching out to us because of (in their view) excessive traffic from our Moodle environment, and it was because of over-aggressive fetching from the rss client.