BrushLabels significantly decrease editor performance

Hello team,

Recently I found an issue with brushlabels, which is that a few brushlabels can decrease the annotation performance. The operations on editor is very laggy. Submitting/Updating takes more than 5 seconds and some times the how browser tab freezes.

Here is how to reproduce it:

Install latest version (1.16.0) with pip: pip install label-studio
Create a project with only one task (image). Define label config with a very simple BrushLabel:
```

Choose Label

```

Now start to annotate, add 5 or 6 annotations. Then click "Submit", the browser freezes for 10 seconds and then finally the task gets submitted.