Time limit.

[Ud71p]

Like in chess and many other games, playing with clock is a good way to equalize how much people think. Then if somebody takes 5 minutes per move, they run out of time and loose. It makes playing more fun, because the opponents cannot leave the game, but need to stay engaged and play.

In fact even a clock without any limit (just showing used time for each player) is good, because then people using much more time can see that, feel a bit guilty, and start moving faster.