This is an entirely uneducated GUESS on my part.
When you do the DFU restore...
- it takes only as long to download the DFU "installer" as your internet connection will support (that is, even a 12gb file may download in a few minutes)...
then...
- The DFU "installer files" might themselves are "pre-configured images" that can be written "en masse" to the target Mac's drive.
That means that all Apple Configurator does is "set up" the internal drive with pre-configured partitions/containers, and then "pours" a single, giant install file into each one, instead of writing tens of thousands of "individual files".
- The high speed of the internal drives (particular on MacBook Pros) makes the "copying process" go quickly.
- Some final configuration and "touch ups", and the install is done.
Or... to put it another way...
There's less "installing", and a good deal more "[only] copying".
Again, this is a complete GUESS and I could be totally wrong.
I welcome correction from those "in the know".