Is it possible to update a customField for multiple people with one API call?

We have added a new customField to FUB. We have around 40K leads on our CRM and need to add data for this customField to FUB.
Is there a batch update API available for the /people resource or we have to do this with individual API calls?
Another issue we are facing is some of the updates we made to add this customField using /people/put seem to not be visible on FUB despite getting a 200 ok response. Is there a delay associated with the /put/people call?