copy transfer rate in my virtual machine is (seriously) 14.6 KB/Sec. !! Perfoming the same copy operation in my host OS it's 379 KB/Sec.
Well, to rule out the basic topics, is your host OS having USB3.0 while your VM is running USB 2.0?
Apart from usb copy, do you face this issue even with network copy difference between host and VM, this will rule out the disk access problems?
Again, config details might be needed to ascertain