Turn based games can be very slow if people are coming from drastically different timezones, ensuring that you essentially are unlikely to get more than one turn per day. During the creation of a user turn-based game you should be able to specify a region for which the game is optimized.
Even better would be the ability to set up a schedule for minimum per-turn times during wake / sleep periods. Say 4 hours before a turn is due during certain hours of the day (for a given time zone) and 12 hours for other parts of the day.