On March 31st, ACE became mandatory for most entries into the United States. In general, the transition has been smooth, but there were slowness issues on Tuesday March 4th that resulted in CSMS messages (16-000270 and  16-000271) from CBP.  This was a result of the Census Bureau adding a group of exporters to the system.  The issues were resolved and CBP is fully committed to increase system response times.