On average, yes.  Having a bad employee in the ranks brings the entire team down and lowers morale, as the bad employee is usually not very happy or the source of positive energy.  Think of a fast food restaurant.  The speed of the service will be determined by the slowest member of the team, regardless of how fast everyone else is.  The faster workers will have to slow down to give the slower employee time to catch up or wait (making the customers wait) while he or she catches up.  In many cases, the service would speed up tremendously if the slower worker wasn’t there at all and the faster team members divided the duties among themselves.

Don’t be afraid of holding all your team members accountable.