Are you pressing the button to start and externally timing the process of arriving at a usable desktop and have you explicitly disabled fast boot? This isn't a new feature in windows 10.
Internally my computer tells me the process takes about 5 seconds from the OS start to graphical environment but in reality there are several steps. For example this doesn't account for the period of time between hitting the power button and the OS itself starting to run, entering full disk encryption password, unlocking volume.
I would be surprised if a full restart actually took so short. Maybe not loading a menu or unlocking a volume is sufficient to explain the difference?