Symptoms
A custom-built view was used to list all items from a SharePoint list. While the auto-generated SharePoint views were very fast to load all items in the list, the custom view was very slow (15 seconds to display only a subset of items).
Diagnoses
The default method selected for querying the SharePoint library SmartObject was "Get Documents", which also gets the files themselves.
Alternatively, one can use the "Get Documents Metadata" method to retrieve only information about the Documents, which is a significantly smaller amount of data.
Resolution
Modify the view to use the "Get Documents Metadata" method.