This is annoying, I also got problems.
First, my computer started up fast with my x299 Asus Tuf Mark 1 and Thermaltake ps 3 core I got yesterday.
Later on, I removed the cable because I needed to put in a USB cable on the motherboard. Then gently add the PCIe cable back again.
This time my boot hanged and the motherboard complaint about VGA. After reading some posts and saw a youtube clip regarding almost the same problem I took it out and put in again and straighten it a little bit.
Now my computer starts but the boot does not hang but something happening, it takes time to boot u