The output of the dongle in it's normal DVB-T mode is an MPEG-2 Transport Stream. The maximum bitrate of DVB-T is 31.67 Mbps or ~4 Mbytes/s. It's thought that the USB interface was designed to support that rate and not a whole lot more (3.2 MS/s would be 6.4 Mbytes/s since it's 2 bytes per sample).