Enunciation
Enunciation is the act of speaking clearly and concisely. The opposite of enunciation is mumbling or slurring.
Enunciation coupled with slow speech is often used as an insult. For example "Can-you-understand-the-words-coming-out-of-my-mouth?" spoken slowly, clipped and each word clearly emphasised in response to the person not understanding. The insult is a slight towards the person's ability to understand the language or 'basic' ideas.