The Guru mantra or any mantra always gives its results as an outcome of its vibrations. That is, the mantra affects through its vibrations. Again, different mantras emit different vibrations which are dependent on the pronunciations. Hence, if the pronunciations are not accurate, the quality of the vibrations is not the best, and the end result is a lessening of the overall fruit. Hence, when the mantra is recited the pronunciations must be as perfect as possible. The repetition of a mantra should not be so fast that the pronunciations are incorrect; at the same time not so slow that it takes too much time.
Shri Dhyanyogi Madhusudandasji