Can't speak or speak less

 Can't speak much and don't speak much aren't the same. 

Some people hesitate, fear, or are too shy to speak up. 

Others speak less because of their personality; they communicate only what is necessary, avoiding unnecessary chatter.

Some shut themselves down when they feel mentally low or in anger. 

Some have physical or mental disabilities that make speaking difficult. These can range from minor reasons like bad breath or crooked teeth, to hiding an accent, struggling with a new language, or having physical or mental limits. 

Another group speaks less because they don't know much or have done something wrong. 

So, when you meet someone who speaks less, it doesn't mean they have a problem preventing them from working. Not all employees need to be talkative. You want someone who will do their job honestly; that's the backbone of your company's success.