USB 3.0 Bus runs only at USB 2.0 speeds?
I would suggest making sure the USB pen drive is really fully plugged in (apply a bit of force). Then reboot the computer while the USB drive is plugged in, and the drive should come up with 3.0 speeds.
The reason behind this is that if you do not fully insert the drive, or you are too slow to insert the device in the port, it can be detected as a 2.0 device. This seems to be a general issue for the SanDisk Ultra product that also Windows users are experiencing.