This should only take a minute of your time. It's an easy test if you have a newer ASUS MB and an External USB Hard Drive.
I am doing this on a Z590 system but I suspect it may also happen on Z690 and Z790 montherboards. If so it's a real bios bug.
I have tested this on a Maximus XIII Hero and a Z590 Gaming motherboard.
ASUS wants to RMA both boards but I doubt it's really a hardware issue and likely BIOS bug.
Steps to test.
Shut down the system.
Disconnect ALL USB hard drives.
Now time just how long it takes for your POST (DEL / F2 to enter BIOS) screen to show from when you press the power button.
Now power down the system again.
Reconnect all your USB external hard drives. Mine are Segate between 4Tb and 10Tb standard physical drives. Not SSD drives.
Now time just how long it takes for your POST screen to show up again.
Did the time it takes the system to POST almost double?
My system reaches the POST screen in about 15 seconds with no USB drives connected.
Roughly 30 seconds once a single USB drive is connected. With all my USB drives connected the system can take from 1.75 to 4.25 minutes to reach the POST screen.
My older ASUS systems do not seem to have this problem no matter how many USB drives I add.
Please provide details as to what board you have and what brand of USB drive you tested.