Testing a little more, the transfer speed between the desktop's black USB ports and the portable drive is only slow (around 2 MB/s) in one direction, from the desktop to the portable drive. From the portable drive to the desktop, it's about the same as before, around 25 MB/s. With the laptop's 3.x USB ports, the direction didn't seem to matter AFAICT.
BTW the portable drive's USB port is labeled SS and colored blue, so it's definitely USB 3.0.