Super Interactive

Done!

I created a work-around by caching the feed as a transient. So instead of:

$feed = fetch_feed( $feed_url );

You would do:

// Require for dealing with feeds
require_once ( ABSPATH . WPINC . '/class-feed.php' );

// Check if feed transients exists
if( false === $feed = get_transient( 'foo_transient_key' ) ) {

   // If not, fetch it
   $feed = fetch_feed( $feed_url );

   // Store as transient for 4 hours
   set_transient( 'foo_transient_key', $feed, 4*60*60 );
}

// You now have an object $feed just like you did before

Now you’ll only fetch the feed once every 4 hours, and W3TC will cache your page the rest of the time.

This seems to fix the problem. I always thought fetch_feed does its own transients/caching though. Either way, it’s still weird W3TC skips entire pages for one simple fetch.