Spike: Investigate performance issues when loading UI identities

API endpoint: `/api/v1/environments//identities/`

For environments with millions of identities, it can be very slow to load the UI, even while it's paginated.

Possibly [this sorting](https://github.com/Flagsmith/flagsmith/blob/b135023160873dbb5feed2e00097174afa49bc8e/api/environments/identities/views.py#L65) could benefit from a binary tree index on `Identity.created_date` exclusively. This spike is to coordinate an investigation and confirm this is the case, or point somewhere else.