This could be the result of a large amount of data within some of the batch and audit tables that are used by the process.  The accumulation of millions of records in these tables could result in an INSERT taking a fraction of a second longer.  This would be noticeable for imports that are processing a large number of records on a regular basis.  Steps to resolve would include:
  1. Delete committed batches that you no longer intend on reviewing.
  2. Utilize global change to delete audit data on a regular basis (PHONEAUDIT, EMAILADDRESSAUDIT, and ADDRESSAUDIT would all be used by this batch if auditing is enabled).  Additional information on deleting audit data:  https://www.blackbaud.com/files/support/helpfiles/crm/us/40/Content/ADMINGCDeleteAuditTables.html
  3. Consider configuring the batch template to 'Delete batch after committing' if your organization does not regularly review committed Constituent update batches.