Document Approvals for Sales orders over credit limit, approval email sent but approval record NOT added

We are using NAV 2009 R2, Standard Document Approval setup for sending over credit limit aproval requests to a credit controller. As the volume increases we seem to get some record locking contention on the Approval entries table, which prevents a new record being added. The approval request email is sent, but the becasue the aproval record table is not updated, the approval response to change the SO status from Pending Approval to Released (approved) is not available to the Approver. Is there a Known performance issue with the document approval process, or a known record contention issue?