I was having same issues with my 950 pro.
go into the uefi, make sure csm is set to disabled
then secure boot to uefi.
To fresh install on win 10 i had to disable csm and turn secure boot to other os then after the installation, before windows configures, go back into the bios and changed secure boot to windows uefi,
then i downloaded samsungs nvme driver.
My boot time is 8.83 seconds and i have video to show. from when i hit the power button to booted into windows. before i changed the secure boot to uefi i would get 2 splash screens so it literally took more than double the time.
Hope this helps. I know it bothered me like crazy. i was like my 850 evo boots faster than this.
other than that, im not sure what else will factor in.
i have herd one person say make sure ahci is disabled to use nvme however im not 100% sure my mobo supports nvme. i have the asus rog maximus viii hero.
if anyone can confirm the ahci being disabled method thatd be awesome.