How can i optimize mu voxel engine with Babylon.js

I'm working since this morning on a little web project (you can find it here) : build a small voxel engine (with textures like Minecraft) with Babylon JS.

I made a small world (size 50 * 50), without blocks below other (just the first layer to walk). I made a similar project with Java, how can i optimise my project ? Because it's lagging.

With Java, i had to :
- Disable the display of inner faces of cubes (I know "backFaceCulling = true;" but it's only working on blocks that haven't got one image per faces but one image for all faces).
- Disable the display of faces when a block is right next to it.
- Use VBO.
- Make a big object grass(composed by all my grass block than make multiple grass cube).

Do you know how can i do this 4 points ? (and oter way to optimize) ?
PS : All of my code source is in this main, do you wan't me to write it here ?