Any time a data source field is referenced by a block, that block will not render until all the referenced data sources are available.
Being available means that if the data source has never been used or has had a filter or sort order changed since it was last used, it will to be retrieved again by the app.
Occasionally you want to have a dependency on a data source that is not displayed by the block itself.
In every Block a field, "Other Dependencies" (under 'Data Management') can be used to express that dependency.
To do so, use the editor and insert a reference to any field in the data source, like "[[ds:/source/item/id]]". You may reference as many data sources as needed by placing an expressing referencing each one in this block.
ACTIONS DO NOT AUTOMATICALLY LOAD DEPENDENCIES
You absolutely need to refer to a Data Source you plan to use in an action before using that action.