Currently there is a mis match between API and SPI on this as fieldDefinitionId is exposed in SPI, but fieldDefIdentifier is needed in API. this causes the following load to occur in a loop in ContentService:

                    "fieldDefIdentifier" => $this->persistenceHandler->contentTypeHandler()->getFieldDefinition(
                        $spiField->fieldDefinitionId,
                        ContentType::STATUS_DEFINED
                    )->identifier

SPI should either get this in addition, or fieldDefinitionId should be replaced by fieldDefIdentifier.