Document ID generation

During bulk migration of documents using PnP PowerShell, its is noticed that Document ID is not getting generated for many documents and gets generated for only few docs.

Do we need to place delay in script for Document id to be generated?

Do we need to set any parameters for Document ID to get generated before next file upload .

Is the service so slow as we require to capture the Document ID

Does doctype matters in associating Document ID? Does check in /checkout option matter in this as it is defaulted to check in