NEW YORK — America’s largest companies added women and minorities to their boards of directors at a faster pace over the past two years, a period when sexual harassment scandals thrust workplace equality into the spotlight, a study shows. The rate of change remains slow, but minorities are making visible gains for the first time in many years, according to the 2018 Board Diversity Census, released Wednesday by the Alliance for Board Diversity and the consulting firm Deloitte. The number of African-Americans on the boards of Fortune 500 companies rose for the first time since the census began tracking the number in 2010.