Personalization changes require an API callout to Sitecore CDP for each Guest updated. Due to Salesforce limits it may not be possible to make all updates immediately if a large number of records are changed at once, CDP4S handles this with an update queue that is processed in the background by a scheduled job.
The updates to be sent are created as CDP4S Queued Update custom objects containing the required data. You can view the currently queued updates in the CDP4S App tab. These records are removed once sent.