Why do you think the wind speeds on the gas giant planets are so much greater than the wind speeds on Earth?
Mainly due to friction caused by land and seas.
Wind is caused by differences in air pressure, which is caused by uneven heating and cooling. Gas giants have not only the heat from the sun but also their own internal heat. Jupiter for example has more of its heat than it receives from Sun. While earth has land and sea, which causes friction to wind and latter slows down, this is not the case with gas giants. Hence, wind speeds on the gas giant planets are much greater than the wind speeds on Earth.