Too long? It should be a matter of milliseconds...
At any rate, I don't think there's another way of doing it using a script (maybe only by exporting the data to an FDF file and then importing it in the second file), after having created the drop-down field.
Please share the code that you are using to re-create the dropdown control. As try67 said, it should be fairly quick to do this, so maybe you are doing something either very inefficiently or wrong in your script. How many items do you have in your source dropdown control?
It's fairly simple to copy the properties of a combo box from one document to another but you'll need to "disclose" it first. You'll also want to use app.openDoc with the hidden attribute set to true, populate the combo box while the document is hidden and then expose it, that way no one see the lag if there is any.