"Play After Synthesis" mode means that, after the [Play] button is pressed, the whole track will be synthesized before actual playback starts. You should set "Play After Synthesis" mode for tracks you have finished to optimize the CPU workload.

In the "Track Manager" window, you can change settings for each track, such as changing the track name, setting play mode, etc.

If you left-click the round button in the "Mode" and select "Play With Synthesis" from the menu, the round button looks like this: . It means that, after the [Play] button is pressed, the track will be played while the synthesis engine synthesizes the waveform. If you select "Play After Synthesis", the round button looks like this: . It means that, after the [Play] button is pressed, the whole track will be synthesized before actual playback starts. If you select "Off", the round button looks like this: . It means that the engine does not synthesize the track. You can select the "Off" mode for a track that you do not want to play at the moment. You should select "Play With Synthesis" mode for tracks you are working on and "Play After Synthesis" mode for tracks you have already finished to optimize the CPU workload.

"Chase Event" should be checked to take into account all control events set before the song position. You should usually check this.

Please also try the followings:

• Increase "Pre-Send Time" in a dialogue displayed by selecting [Setting]->[VOCALOID MIDI]->[Option] from the menu.
  

• Increase "Buffer Size" in a dialogue displayed by selecting [Audio]->[Device] from the menu.
  

• Increase the buffer size or latency of the sound device (if possible).
  
• Set the sampling rate to 44.1kHz.
  
• Reduce the number of "Play With Synthesis" tracks.