No, it's cached since 3.6
I see you are still using the 3.5 branch: then it uses a query (so you might want to upgrade).
Quote:
Originally Posted by Kier
Cache of Syndication Data (external.php)
Data outputted by external.php (RSS feeds, for example) will now be cached for a specified amount of time. This defaults to one hour. This significantly improves performance on large boards.
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