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One can say that Solomon Asch experiments on social conformity are great illustrations of the benefits of free speech. Diverse opinions discourage group think and encourage critical thinking and discourse. That works even if the opinion is bogus, since in that case it may encourage the individual to avoid adopting an overwhelming majority point of view. As you mentioned it is indeed a slower process, but a much more fault tolerant one. The simple analogy is the slower, complicated democratic process vs fast, simple autocratic process.