On my RPi2, I have measured exactly 5 seconds between :
and
Is there a way to optimize this before-kernel-boot-time in config.txt or cmdline.txt?
I already have set boot_delay=0, that saves 1 second boot time, compared to default value boot_delay=1 (see http://elinux.org/RPiconfig)
I doubt that there is much to gain by tweaking the config files to speed up the GPU boot strap process. To my knowledge cmdline.txt is passed to the kernel only so it will not influence the pre-kernel-boot-time. config.txt on the other hand is already used during the boot so it at least thinkable that overclocking settings for GPU and RAM frequency might speed up things a little (or just fail horribly).
Other than that it would seem to me that changes to the system would require a rewrite of the different stages of the bootloader (see here). But whether this would really speed things up or not is questionable and depends on how heavily that code is already optimized and what effort one likes to spend on doing it better...
