Fastboot isn't about speeding up the boot process in some magical manner by the Intel Linux graphics driver, but rather it's yet another take at trying to streamline and beautify the Linux boot experience. The goal of all this work is to just preserve the frame-buffer as setup by the BIOS (or GRUB) to avoid any flickering at all until the X.Org Server has started. Depending upon the hardware/software configuration, the X driver may also be able to start without causing any flickering / mode changes to occur, which would then mean this is truly a flicker-free boot experience.
Linux graphics driver developers for years have been trying to achieve a flicker-free experience... Originally it was all supposed to be magical with the kernel mode-setting (KMS) drivers, then with Plymouth on KMS, and various other attempts to not cause the display to flicker at all while Linux is booting. For over four years there have been many articles about flicker-free on Phoronix when it comes to Linux graphics drivers. Rather than calling it Fastboot, it could be called YetAnotherTryToAvoidFlickering. With this flicker-free goal going on for several years the amount of resources devoted to it is rather surprising when consdering that the open-source Intel Linux graphics driver is now slower than the Intel Windows driver and the OpenGL support is years behind.
These 43 patches authored by Jesse Barnes and others is the latest flicker-free attempt for the Intel Linux graphics driver by just trying to preserve the original frame-buffer for as long as possible. 25 of these patches lay the ground work for a new type of GEM object and allows for it to be preserved after the GEM memory manager initializes while the rest are about extracting the current mode configuration.
The Intel Fastboot patch series can be found on the intel-gfx mailing list. The many patches that affect hundreds of lines of code with the Intel DRM driver can be found via CGit as well for easy, web-based viewing.