Improve perf

[francisrstokes]

• This PR only introduces changes to documentation.
  • Please include a summary of changes and an explanation
• This PR adds new functionality
  • Is there a related issue?
    • Please note the related issue below, and how this PR relates to the issue if appropriate
  • Does the code style reasonably match the existing code?
  • Are the changes tested (using the existing format, as far as is possible?)
  • Are the changes documented in the readme with a suitable example?
  • Is the table of contents updated?
  • Is the index.d.ts file updated, using the same (or appropriately modified) description as in the readme?
• This PR introduces some other kind of change

A pretty obvious improvement here - using an object to keep track of field names instead of an array. I noticed it was performing slowly on a pretty straightforward structure with thousands of fields, and realised it was spending a ton of time traversing arrays every time a field was added (and getting longer each time) 🤦‍♂️
No breaks in the user API, so this is just a nice "win".