Here is a place discussing this: http://superuser.com/questions/808279/why-does-copying-individual-files-take-so-much-longer-than-one-large-file

Moreover, another take on file size and performance: http://gamedev.stackexchange.com/questions/20247/why-are-big-files-better-than-small-files-for ...

The game has a huge number of tiny files for its relative small total size, and they can negatively impact disk (de)fragmentation.
Can the game files be packed in larger files for the future ea builds or the release version?

Thank you.