Is there a table for the cross-reference to resolve the 'low level return values'?  (I forced a failure, return = '-2', but not sure what that means)

You can use the line-by-line monitor to see where the query is spending most of its time.

http://docs.intersystems.com/cache201513/csp/docbook/DocBook.UI.Page.cls?KEY=GCM_monlbl