Is the stick USB 2.0 or USB 3.0? If it's USB 3.0, make sure it's plugged into a USB 3.0 port.
You're right, though. 15 MB/s is USB 1.1 speeds. USB 2.0 is 480 MB/s, and USB 3.0 (or USB 3.1 Gen 1/USB 3.2 Gen 1, depending on who you ask) is 5 Gbps/10 Gbps.