Have you tried looking at the query plan for your query to see which indexes it is using and whether these appear to make sense?

It appears that the generated query will often ignore the sessionId index as below:

%IGNOREINDEX Ens.MessageHeader.SessionId

Override the  AssignOneSetting method in the adapter your operation is using to get the setting value from somehere of your choosing.

It's also possible to override the adapters and get the data from your own data within the namespace.