Most of the experienced chess players opt to play faster games as it is easier for them to play fast as they would have played a lot of games and they might feel very confident about their moves. Moreover the speed with which they see the tactics of chess is also more and their positional game play is also quite good.
Even if they are quite experienced in their gameplay there may be a few subtleties which even they might miss. Very strong players take advantage of these inaccuracies to their advantage and may also convert it to victories. Also younger players still need to grab new ideas and experience the real chess so they usually are unable to make quick decisions as they need more experience in playing such positions.
Playing longer formats help them register these ideas in their mind and the next time they encounter it, it becomes easier to play on the board. So wherever one person plays, experience is what helps them play fast chess.
