Use case
The Microsoft OpenXml SDK relies on the System.IO.Packaging API to generate Microsoft Excel files. This allows adding hyperlinks to Excel columns, but with the current resource consumption this is impossible when you have an excel sheet with about 100.000 records that each has a link to some additional record information - for instance a deep link to a HTTP resource. This API uses PackagePart.CreateRelationship inside the System.IO.Packaging stack.

Background
When adding hyperlinks to Excel cells, you have to call AddHyperlinkRelationship on the OpenXmlContainer. This will then follow the stack in System.IO.Packaging:
PackagePart.CreateRelationship -> InternalRelationshipCollection.Add -> InternalRelationshipCollection.ValidateUniqueRelationshipId -> InternalRelationshipCollection.GetRelationshipIndex.

Problem
ValidateUniqueRelationshipId will call GetRelationshipIndex to ensure that the relationship id is unique, and it loops through all the links each time it is invoked. As the identifier list grows it will (obviously) become exponentially slower. Backing this identifier store by something similar to a HashSet would greatly improve the lookup time.