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Google is incorporating site speed as one of the over 200 signals that we use in determining search rankings...
...fewer than 1% of search queries will change as a result of incorporating site speed into our ranking.
...please don't worry that the effect of this change will be huge.
Google incorporating site speed in search rankings [mattcutts.com]
Site speed is at most a small factor - something like a tie breaker.
and if the webmaster tools graphs are anything to go by
just a way to reduce the load on their bots?