In order to allow Time Steps and Build Cycles to process more quickly, you need to minimize the number of calculations to make. Several common ways to do this are:
Turn OFF orbital motion for planets and asteroids.
Start the game with ZERO Non-Player Races.
As time goes on, and more and more of the galaxy is explored and generated, the amount of processing required for a given build cycle will grow, despite your best efforts. It is my understanding that this provides many players with their "Game Over" condition; when turns take an unbearable amount of time to process, it is time to scrap the present game and start a new one.