Significant performance degradation when using react-final-form-arrays

I have a lot of forms in expansion panel and all of them have arrays in them. I am seeing significant performance impact . On further investigation looks like Chrome performance timeline tool is pointing towards ```react-final-form-arrays.es.js:289``` which is significantly slowing down user experience . From the time user clicks and the changes show up on screen is a lot.

@erikras Do you have any thoughts on how I can improve the experience.

Reproduce the slowdown with multiple forms in expansion panels, each containing arrays, then record the interaction in Chrome's performance timeline. Inspect react-final-form-arrays.es.js:289 and compare the delay between a click, the array change, and the updated display. Done means the reported interaction lag is reduced and the affected scenario remains functional.

Written by the indexing model from the issue text.