Hi, this is quite common unfortunately and has nothing to do with the interface (USB) speed but rather with the flash memory speed. The USB 2.0 has a maximum theoretical speed of 480 Mbps (Megabits per second) which is 60MBps (MegaBytes per second, 1 byte = 8bits). But that is the “speed limit”, not the actual “speed” things necessarily work at. The bottleneck in this case isn’t the interface but the actual flash memory chip that is slow (also cheaper) than most.
Also, it’s quite common for write speeds to be different from read speeds, with the read speeds being quite superior to write speeds.
So, unfortunately, what you describe is nothing unusual or strange.