I remember reading a best practice somewhere (not quite sure where) that if possible an action should return "Success" by default rather than "Process complete but withfailures", as failure dump generation lengthens the import duration.
Just wondering if this is the case and if anyone's aware of any correlation between the size of the failure dump and the import duration?
Yes, you are correct, all actions should complete with a Success for two reasons:
you know all data is correct and if there is, in fact, an error/warning, the import needs further investigation
having the system create a debug log does in fact lengthen the action time because it has to create said debug log. The length of time it creates the debug log is dependent on the number of issues the import has found.