A data input/output (I/O) device includes a plurality of data units and an I/O assembly. The plurality of data units is coupled to a global I/O (GIO) line through corresponding local I/O (LIO) lines and configured to receive or transmit a plurality of data groups through the corresponding LIO lines. At least one of the plurality of data units have a different operation speed. The I/O assembly performs serial/parallel conversion operations on the plurality of data groups including a high-speed data group and outputs results of the serial/parallel conversion operations. The high-speed data group is output from the at least one of the plurality of data units having the different operation speed.