The error message about "this device would perform faster if connected to a USB 2.0 port" indicates that it's been enumerated wrong and it's not running at USB 2.0 speeds at all (or really is plugged into a USB1.1 port/hub); this is nothing to do with the fact that Rockbox's actual file transfer speed is slower.
I suspect the device isn't installed properly in Windows; try uninstalling it from device manager then disconnect and reconnect it, or just try a different port.