I have two of them. They are the slowest boards I've seen with respect to the time it takes to go from power-off to login screen, even with an SSD, but I've never seen 15-20 minute delays. I have two suggestions.
First, unplug the Ethernet cable and remove / disconnect everything, and I mean everything, except for the bare essentials. Remove all add-on cards and disconnect all drives. If you have a CPU with on-chip graphics, remove the video card and use the on-chip graphics. If the problem disappears, add each part back one at a time to identify the defective one.
Second, move to a different BIOS as a test, but do remember that moving beyond 0028 is permanent (and not recommended by me). For example, if your board has a BIOS of 0028, move to 0027. If your board has a BIOS of 0042, move to 0039. My theory here is that something in BIOS was over-written and this will refresh it. After you fix the problem, return to your original BIOS.
Thanks for the suggestions. My case came with a Bluetooth adapter (that I have never used). After disconnecting that, the computer started normally.