It may depend on computer CPU usage. However, playback may be improved by checking the buffer size of the Audio Interface.
[Setting] > [Preferremces] >[Audio] >[Buffer Size]
[Preferremces] >[Audio] >[Buffer Size]
Another factor is the "Offline rendering time". VOCALOID6 synthesizes the vocal sound right after each edit. This is performed for all notes in the part. The sound stops until rendering is completed. Shortening the part size reduces the rendering time.