This is another symptom of depression as identified in The Diagnostic and Statistical Manual of Mental Disorders - first published in the 1860s.
However, as we know by now, depression is always contrary so in some sufferers - instead of a slowing down there is a speeding up and this is known as psychomotor agitation. This is characterised by unecessary, fast movements such as pacing or toe-tapping. In these sufferers, talking can also be accelerated. I suffered from agitated depression and I used to pace the house - completely unable to settle in a chair; I wasn't a great talker though.