This depends on what type of CD you are creating.
If you are creating an MP3 CD, click on the MP3 CD wizard icon on the right side of the "Arrange Songs List" and click "Next". Then select "Fast" and click "Finish". Note that in fast mode, song/track order cannot be preserved and all conversions will be skipped.
Otherwise, when you click burn, make sure that the burner mode is set to "Burn CD!" and that the burner speed is set to "Max".