It is named as “Insider Fast Configuration update”. I received this update on my Lumia 640 XL and Lumia 1020 running Build 10136. It takes around 10-15 min in downloading the update package and around 15-20 min in installation and data restore. It is tough to tell what it changes or fixes but it may be an update to fix some hidden bug or may be it is readying the devices for future Windows 10 Mobile builds.
We will update this article if we come to know what it does.
Thanks Amith for the tip. Cheers!!