Entity Groups Query Performance

[savilak]

I have an ACCOUNT entity and then a child CONTACT entity. (1 Account has many Contacts).

I query to get 50 contacts:

DatastoreService datastore = DatastoreServiceFactory.getDatastoreService();

Key accountKey = KeyFactory.createKey("Account", uid);

Query query = new Query("Contact").setAncestor(accountKey);

FetchOptions fetchOptions = FetchOptions.Builder.withLimit(50).prefetchSize(50);

QueryResultList<Entity> contacts = datastore.prepare(query).asQueryResultList(fetchOptions);

In case the Account has

300 Contacts  the query to get the first 50 contacts takes ----> 21  miliseconds

3000 Contacts the query to get the first 50 contacts takes   ----> 234  miliseconds 

10000 Contacts the query to get the first 50 contacts takes  ----> 1307  miliseconds 

Why is this the case? In all three cases I touch only the first 50 contacts.

Theory says Entity Grouping should not affect query performance.

Thank you for your time.