When moving to version 0.15.2 the open3d.io.write_point_cloud function works extremely slow *works fine at 0.14*

### 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).

### Steps to reproduce the issue

I Installed open3d via pip install

created a function of saving ply
```
import numpy as np
import open3d as o3d
import threading

def save_ply(path, idx, points, colors, points2, colors2):
pcd = o3d.geometry.PointCloud()
pcd.points = o3d.utility.Vector3dVector(np.concatenate((points, points2)))
pcd.colors = o3d.utility.Vector3dVector(np.concatenate((colors, colors2)) / 255.0)

o3d.io.write_point_cloud(path + "/" + str(idx) + ".ply", pcd, write_ascii=True)

while True:
t1 = threading.Thread(target=save_ply,
args=(
self.record_folder_name, self.idx, self.points, self.colors, self.points2,
self.colors2))
t1.start()
```

### Error message

```shell
No error message, but the visualizer just slows down really fast and crashes.
```

### Open3D, Python and System information

```markdown
- Operating system: Ubuntu 20.04
- Python version: Python 3.8
- Open3D version: output from python: 0.15.2
- System architecture: x86
- Is this a remote workstation?: no
- How did you install Open3D?: pip
```

### Additional information

I have made a visualizer that run over plys and display a "movie" of pointclouds.
I record that data by saving it on a thread, and it works fine at 0.13 and 0.14 of open3d, but for some reason that is not familiar to me, when I bump up to version 0.15.2 it crashes, and when I go back to 0.14.1 it seems to work fine..

If you need my visualizer code as well with some data to run over, I would love to share aswell, but I believe that my code that I showed in the step by step should be enough to recreate the error.