Datalayers: Sequential or Simultaneous?
The Logi Server Engine will usually run multiple datalayers in the same definition simultaneously, in separate server process threads, to provide the best performance. You can see this happening by examining the Debugger Trace Report: each datalayer running in its own thread will have a "Thread for Datalayer ID" reference.
An exception to this is when datalayers are used under multiple Local Data elements. The Local Data elements (and their datalayers) are run sequentially, one after the other, in the top-to-bottom order of the elements in the definition. This is done so that the data from one datalayer can be used as criteria in a query in the next datalayer. There is no way to avoid this other than planning your application carefully to minimize the use of multiple Local Data elements, if data retrieval performance becomes an issue.