Starting from version 3.4.0, you can adjust the MMD play speed while keeping the music quality still as good as possible. This allows you to slow down/speed up the dance with still acceptable music quality in sync.
3.4.1 updates:
Fixed problem that music pitch optimization did not work when adjusting music speed with Audio Atom option on in MMD Player for spatial sound effects.
Added more options like pitch and volume along with the music speed option so user can further customize the audio pitch shift.
How to adjust the play speed?
You add the MMDPlayer plugin and load a VMD folder to play as usual, then you add the SuperMode plugin, you will see a slider to adjust Music Speed when clicking on the [VMD]Person node in super mode. Just move the slider to slow down or speed up.