The hotter the air is, the less dense it is with oxygen. Also, the high humidity also leads to less oxygen molecules in the air.
Since the engine mixes oxygen and gasoline and ignites it to make power, less oxygen means less power.
Turbo cars suffer less from this problem because their turbos counteract the less oxygen dense air by spooling the turbo's longer until the required boost is reached. This may increase turbo lag, but power will still be good.
Something to look forward to in the new M3!