This is a lesser-know fact, but all the USB ports on your computer are not treated with equal priority. As a rule, the USB devices that are plugged into the USB root (the ports that are directly connected to the USB controller on your motherboard) are treated with a higher priority and are the first to get detected and powered.

Therefore, if you tend to boot your system from a USB device, it is preferable to plug the device into one of the USB ports on the rear of the computer as opposed to an external USB hub. Even with the USB ports on the motherboard of your computer, there is a detection priority through which USB devices are recognized: the back panel 4-port USB stack is the first to get detected, followed by two ports that usually terminate on the front panel of your cabinet, and finally the stack of two USB ports on the rear of your computer.

Keep this in mind while using bootable USB devices and you can reduce the time your computer takes to boot.