I understand that when you have your USB 3.0 device connected to an USB 3.0 port, the POST sequence slows down a lot.
The reason why the POST sequence slows down is because the motherboard is trying to recognize the device that you have connected and it is checking for bootable options.
I will report the issue that you are describing to check if there is anything we can do from our side to improve the compatibility with the removable hard drive when being used in a USB 3.0 port.
Currently I recommend you to update the BIOS as mayor changes have been made and there are several coming that may help with the issue that you are experiencing.
Also, you can check if disabling the boot to USB devices on the boot tab will help you with the boot time.