ModelBuilder#createRect will create a new model for each rectangle. When rendering a (part of a) model instance, it implies a draw call. Therefor ModelBuilder#createRect is extremely inefficient. It is better to combine multiple rectangle into a single (part of a) Model. This can be done using:
modelBuilder.begin();
MeshPartBuilder mpb = modelBuilder.part(....);
mpb.rect(...); // first rect.
mpb.rect(...); // second rect.
// etc.
Model model = modelBuilder.end();
Note that this is still not efficient enough for e.g. a voxel engine. If you are aiming to optimize for voxels, you'll probably want to build the mesh (after frustum culling and depth sorting) in a custom RenderableProvider. Here's an example: https://github.com/libgdx/libgdx/tree/master/tests/gdx-tests/src/com/badlogic/gdx/tests/g3d/voxel