The data transfer speed will be affected by the physical electrical connection at the USB port and the load the CPU is under. If the computer was doing something else like an automatic virus scan or if the usb did not have a good connection this would slow it down. Also check that all ports on your computer are USB3 compliant and you have the latest drivers. Some ports may be faster than others. Remove the USB by safely ejecting and try again. Also test the USB on another machine. A USB 3 port should have a blue centre. Of course you can still use a USB3 on a USB2 port but you will need the correct driver.