MMDPlayer 3.4.0 updates:
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.
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.