Overt enunciation will dramatically hurt speech recognition. But so will leaving out great chunks of the words. This is very true with continuous speech recognition. Each word should take an appropriate time to its size. A long word should take longer to say, a short word should take a shorter time to say.
Many people speaking to a computer begin to speak in a very artificial way.
One of the things people tend to do is to give every word the exact same amount of time in which to be spoken. This artificial imposition degrades the ability of speech recognition to operate properly. Many people, even the very well-educated and powerful, consider computers to be an authority figure. As a result, when prompted they will speak. During training this can produce humorous but unproductive behaviors.