Answered on Quora.
Depends why they are speaking rapidly. Are they speaking rapidly because they are nervous? Then that has nothing to do with them being smart, although it is not uncommon for smart nervous people to talk fast. People who talk fast are generally above average intelligence.
If they are talking fast because they are smart, tell them to slow down. It’s rude to talk so fast that no one can understand what you are talking about. It’s a social faux pas.
There are some people who are quite bright in terms of being able to solve a lot of difficult problems in college work but think in a deliberate and rather slow way. These people simply have a different thinking style, but they will get the right answer as often as the fast thinkers.
