I currently have the ASUS X99-DELUXE/U3.1 motherboard and a Samsung 950 PRO 256GB SSD as my boot drive.
Although boot is fairly quick, its different each time. Sometimes it goes more or less straight to the Windows 10 welcome screen but takes awhile, other times it shows an ASUS logo with a loading circle. Its never the same each time you boot or restart the PC.
I have tried the following:
Clean install of Windows 10 Pro
Disable CSM, DDR Training,...