Don't know. Once the form is loaded the table would be in memory so I expect it is fairly quick either way. I assume producing the clone takes some time.
However counting the form's record source seems a more general and consistent solution to detecting and dealing with an empty form which is usually the ultimate purpose of this kind of task. Exactly the same line on any form rather than having to refer to a particular table or query.
I wonder what the comparison would be if the record source was a parameter query. Would the parameters be requested again for the DCount or would it just count the current incidence of the query?