USB hub speed vs actual transfer speed
Solution 1:
The BRCM20702 is the Broadcom USB hub for the Bluetooth controller. So it only needs 12 mb/s speed.
It has little to do with the external USB 3 ports except that it uses USB protocols to communicate with the CPU.
Connect a disk drive and using System Information inspect the USB section. You will see there that you have USB controller and hub used for the USB 3 ports running at 5 Gb/s.