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If a language doesn't carry much meaning per syllable, the syllable structures will probably be simpler and the speaker can speak more syllables in less time. On the other hand, if the syllables are denser and have more information per syllable, you don't have to speak quite as fast in order to maintain your rate of information transfer.
Japanese is an example of a language that has a really simple syllable structure and has "a fast rate of low-density syllables". So I guess it would sound the fastest because it has the highest syllabic rate.
