Summarize the question. (e.g., "How to run reconstruction system with RealSense D435i? ")

### Checklist

- [X] I have searched for [similar issues](https://github.com/isl-org/Open3D/issues).
- [X] For Python issues, I have tested with the [latest development wheel](http://www.open3d.org/docs/latest/getting_started.html#development-version-pip).
- [X] I have checked the [release documentation](http://www.open3d.org/docs/release/) and the [latest documentation](http://www.open3d.org/docs/latest/) (for `master` branch).

### My Question

I followed the tutorial(http://www.open3d.org/docs/release/tutorial/t_reconstruction_system/index.html) to build a reconstruction system. Everything works well, but I found that customized integration **is much slower than** the integrate function of VoxelBlockGrid

The figure below is customized integration.

The figure below is the function of VoxelBlockGrid.

I want to know the reason, and wondering if there is anyway to solve the problem.
Thanks!