I have a world made of many cubes (like in Minecraft), they have only color (not texture). I am rendering them using OpenGL 3.3 core profile (GLFW3, GLAD, GLM). I am already have done some optimizations:

• using one VBO for all cubes
• not rendering internal faces
• not rendering faces that can't be visible from camera
• rendering faces using GL_TRIANGLES and indexing

But it's still slower than Minecraft with Optifine, but Minecraft is in Java and blocks have textures, there is bigger world (there are entities etc. too)! How can I optimize my program further? I want to make it playable on sloooow machines too :).
Edit:
I think I might do some work with threading, but I don't know what should different threads do.
Edit 2:
It may be something with pre-computing too. Player rarely change the world (only small part of ticks).
Edit 3:
Maybe geometry shader can help?
Edit 4:
I am filtering faces with this code:

// for every block if (blocks[x][y][z] != nullptr) { // nullptr means that block is air bool drawSides[6]; if (camera.pos.z > blocks[x][y][z]->pos.z * BLOCK_SIZE || world.GetBlockAt(BlockPos(x, y, z - 1)) != nullptr) drawSides[0] = false; // don't draw that side else drawSides[0] = true; // draw that side // other 5 sides blocks[x][y][z].draw(sides); }