If your using a USB hub, this can reduce speeds due to the way they redirect data.
To get a little more performance out of a hdd's or usb flashdrive/hdd, if you go into device manager, expand your list of flashdrives/hdds and right-click, choose properties, policies and then tick the box to switch off write-caching buffer flushing.
Please be aware that you cannot just unplug a usb device with this active as you could loose unwritten data, you must use the usb icon found in the system tray and eject the drive.
USB transfer data in burst mode. The transfer rate is not continuous. Those values are peak transfer rate. Nominal transfer rate is well below that. Firewire 400 is better than USB 2.0 since it is constant 400Mbps. eSATA is better too (up to 3Gbps).