Found this guide by Atmel on getting more performance from your code ~
http://www.atmel.com/Images/doc8453.pdf
Some obvious, some small gains, but every byte counts 
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You mean to say people haven’t been using that?
In all seriousness though, that should probably be linked to in one of the starter tutorials, it’s handy.
That said, size is more important than speed, so not all the tips are useful.