Combined Files dialog - fastest size option?

We often use the Combine Files feature (File> Create PDF> Merge Files into a Single PDF) to get a total page count for a large number of files. I'm sure that wasn't the feature's original purpose, but it's a huge convenience for our often-enormous multi-part documents.
 
Naturally for dozens (sometimes even hundreds) of files, the procedure can take a while. Does anyone know which of the 3 ‘File Size’ choices -- the icons at the bottom right of the dialog -- is the *fastest*? Is the smallest the fastest, or does it take more resources to compress? And is the answer affected by whether or not Acrobat is converting non-PDF files vs. just combining PDFs?
 
Output quality is not a factor for this task, since we create these huge blobs only to get a rough total page count (well before the doc is finalized) & then discard them.
 
Thanks for any clues.