Describe the bug

Editing files collections is very slow, by making them almost unusable when you have a lot of entries.

To Reproduce

See attached zipped file.json sample containing 1000 items. Each item has just two fields:

• name
• id

Define a files collection as follow:

collections:
  - label: Files Collections
    name: files_collections
    files:
      - label: Items
        name: items
        file: data/file.json
        fields:
          - label: Items
            label_singular: Item
            name: items
            widget: list
            fields:
              - label: Name
                name: name
                widget: string
              - label: ID
                name: id
                widget: string

Listing is very slow (about 5 seconds). If you open the developer tool and reload the item list you'll see few violations. But the worst performance it happens when you add a new entry by using the Add item button:

• it takes about 3/4 seconds before being able to scroll all the way down to the end of the list;
• it takes about 6 seconds from the time you start writing in a field until you receive the feedback, making the editing of an entry unusable.

Expected behavior

The interaction with a file collection should be much more responsive, even with a lot of items

Applicable Versions:

• Netlify CMS version: 2.12.15
• Git provider: git-gateway/github
• OS: ubuntu 18.04
• Browser version: Google Chrome 83.0.4103.106

CMS configuration

See Above

file.zip