Where does WordPress default SimplePie save cache data?

In WordPress, the WP_Feed_Cache class uses the WP_Feed_Cache_Transient class to store the results into a transient.

So, short answer, either in the database or in the Object Cache if you have a persistent object cache stored.

If you need to cache something for a period of time, use a transient.

http://codex.wordpress.org/Transients_API

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