When a Primary key is added to the source table, the target table does not get updated with this DDL, despite the DDL replication turned on. The symptoms are, UPDATEs or DELETEs using all columns to locate the correct row on target. This results in slowness in posting.
The following steps can be undertaken to correct the structure of the target table:
1. Make a copy of the currently active config file
2. Activate the copy. This will push the object cache of source to the target whereby the target object cache will be updated to reflect the changes by way of addition of Primary key.
From now on, the target will also use the Primary key.