Performance problem with Field or FieldArray during registration and unregistration fields

### Are you submitting a **bug report** or a **feature request**?

Bug

### What is the current behavior?

Check the sandbox below and click on Initialize button and then on Reset.
Slow registration and unregistration bigger amount of fields(500 rows with 3 columns = 1500 fields).
I have used subscription only to 'value' and 'submitting'.
I haven't provided any validation yet but keep in mind that it will be also included for the subscription.
I have tried also Field instead of FieldArray. In the sandbox, it is included as a comment.

### What is the expected behavior?

Better performance.

### Sandbox Link

https://codesandbox.io/s/youthful-morning-msnpk
https://msnpk.csb.app/

### What's your environment?

"final-form": "^4.18.5",
"final-form-arrays": "^3.0.1",
"react": "16.8.6",
"react-dom": "16.8.6",
"react-scripts": "3.0.1",
"react-final-form": "^6.3.0",
"react-final-form-arrays": "^3.1.1"

### Other information

`````` During initialization produce lot of records

I really love the final form. I have used redux-form, formik, react hook form and nothing is so powerful than your library. Just this performance makes me crazy. Thank you for help.