An impact transmission speed data line

Jd on the purchase of a solid-state hard drive 64G of mobile.
Fast, nominal USB3.1. One end of the data line using the normal USB, the other end is a type-c.
After two months of use can not be found where the identified mobile hard disk, a problem is suspected data line.
So buy a relatively inexpensive data line, found to be able to use, but the transmission speed from the original 130M per second (display speed), down to less than 40M per second.
Rate of decline is obvious. Suspect is not high enough standard data lines.
Visible, data lines will also affect the results.