The helper methods GetTopic(), GetAssociation() and so on retrieve all of the data associated with the Topic or Association from either the local object cache or (if not in the cache) from the database. This retrieval requires additional queries and so can have a significant impact on query performance. If you require only a small amount of information (e.g. a topic name and an occurrence value), it is better to construct your query to return just these values than to return the ID of the topic and then use the GetTopic() method and the API to retrieve the required values.