Question On startup, screen doesn't turn on till the PC has reached the desktop

[Raven A]

When I start up my PC, the screen remains black (doesn't turn on). It doesn't show the Windows symbol with the loading circle, and doesn't show the screen where it says "press F2 to enter BIOS". Only after logging in and reaching the desktop, the screen turns on. This takes about 15 seconds. This might not seem like a problem, but I am unable to enter the BIOS: If I repeatedly press F2 on startup, the PC will enter the BIOS, but the screen will stay off. Sometimes when this happens, the "turn on" signal from the motherboard speaker doesn't activate. I have to force shutdown the PC when this happens.

Interestingly, quite recently whenever I played videos of high quality (4K60fps or higher), the PC would stutter and slow down significantly. I rewired the power wires for the CPU, motherboard and GPUs, and that worked. However, the problem above still persists.

The screen is not the problem, as it works well with my laptop. Also, I am using one of my 2 SLI GPUs for output, currently on DP. With the same GPU, the same problem appears when using HDMI. When using an output from my second GPU, the signal never comes through and the screen remains off. When using the DP/HDMI on the motherboard, the screen also turns on when reaching the desktop but no earlier.

Specs:
Motherboard: Asus Z170 Pro Gaming, 16GB Corsair ram
CPU: i7 6700K
GPU: 2x GTX 970 (different manufacturers, one is a Gigabyte windforce and the other is KFA2)
Boot SSD is a 128GB kingston that is about 4 years old
PSU: Corsair CX750M

 

[cherry blossoms]

Fast boot [enabled] appears to be the BIOS default according to your manual. This initializes hardware DURING the boot process, with only basic I/O at first during boot in order to load the OS. [disabled] should cause the system to initialize hardware during the POST, then boot the system.

To get to the BIOS to try changing that setting, you may need to pull your graphics cards, and do a CMOS reset if pulling them doesn't allow the iGPU to display at boot.