OS X Finder: Why does it take less time to copy files in succession than to do so all at once? Example: 


Select 10x2GB files, then drag and drop to another location. 
Drag and drop each 2GB file to the location individually, in succession (10 times). 


I’ve tested this across multiple OS X versions and Apple computers, and the latter example always executes faster.


Why’s the drag and drop sequential fashion faster? 
That said, why doesn’t Finder just execute batch operations in the same manner? I must be missing something(s)…


Note: This is a question that teeters on the edge of not being within the bounds of this site, yet nonetheless, one worth asking :)



The results are the same regardless of the order (batch/manual) in which it's performed.
I think in your example you are copying all files to the same disk. If the finder is trying to write to multiple places (or reading from multiple places) on the same hard drive, it will be slowed down by drive access times. It is easier (faster) to read (or write) one whole file at a time rather than ten files at the same time.

You can observe similar behavior if you are compressing video to a smaller codec. If you choose to export to the same disk that you are reading your source file from, your export will go much slower than exporting to another drive.

You could re-do your test using 3 separate disks (provided those disks were not being accessed across the same bus (USB3/Thunderbolt, etc.). That might be interesting to test.