Simple, commonly-called template functions (the_title, get_permalink, etc.) are all channeled through the get_post function. While get_post implements caching, it caches content before it's been filtered, not after. This means that sanitize_post is called for each and every call to get_post.
I loaded a test page 5 times in a test configuration that had 86 calls to get_post (mostly via get_the_title and get_permalink). Instrumentation results from APD indicate that fully 8-15% (~9.5% avg.) of the page generation time was spent in sanitize_post and sanitize_post_field, even though nearly all of the calls were for "raw" data, and no filtering was actually taking place. I added a simple early bail-out to bypass the no-op actions, and this reduced the hit to .1-.2% across another 5 page loads (no, that's not a typo ;).
I'm attaching a patch to fix this bug, but this is only the immediate problem (get_post appears to be called with "raw" way more frequently then with "display"). Opening another bug on the filtering implementation may be warranted.
