Saki doesn't understand why she was taken to play mahjong but that's where Saki and Nodoka met. Nodoka wins but questions Saki's playing style so she asks for another game.
So in other words, it cuts the time in half.
لِذا بمفهوم آخر، تختصرُ الوقتَ إلى النصف
Entonces, cortan el juego a la mitad.
Alors, ça prend 
deux fois moins de temps…
Allora, così facendo, la partita dura la metà.
Ou seja, o tempo cai pela metade.