Camila Melodia High Def Unrigged MMD Model

You don't need that many levels of subdivision.  Try it with fewer levels.  I think you'll see you can cut the vert count fourfold without affecting the model's appearance.

As you can tell, high poly models are a pain in the ass to work with.

Note that there are still plenty of ways to weight this, regardless of vert count.  Weight transfer (data transfer modifier).  Linear gradients.  Parent to armature with automatic weights.  Most weighting doesn't involve painting individual verts.