Help! Need to know if there's a mental issue or slowness that can cause a speech impediment, such as not being able to form words?

More info needed. That is a very general question. It depends on many factors, including the age and overall health of the patient, the nature of the symptoms, medications being taken, and many others. However, it is safe to say that the development of such a symptom where it previously did not exist warrants an evaluation with a primary care physician.