By default session keeps the data about the whole database structure. If you have a huge number of tables/ fields it takes long to read and validate the database structure every time on session load.
To overcome such situation you can use a special option that significantly reduces the time of processing when you load this session all subsequent times
The option is named “Handle only selected tables in the session” located at the Execution stage.
Select the needed tables in the tree-view, check this option and then (re)save the session (click “Save” button at the Execution stage).
When you load this session next time, it will process only selected tables.
It’s also possible to use this option for any already existent session. Load a required session, check this option and re-save a session (with new name preferably).