As of 4.21 ArcGIS JavaScript made a change to the way it initially fetches and sets up a new FeatureLayer in the API. There is a request to get the total count of records. This can be extremely slow for large datasets and degrades performance rather than improves it for apps.
ESRI should fix the API to make the behaviour opt-in, update the docs and release notes to clarify the trade-offs so developers understsand what it is doing and what is best for their use case.
Once ESRI fixes the behavior to make it opt-in, they can reword the release notes like this: