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__c custom objects containing the required data. You can view the currently queued updates in the CDP4S App tab. These records are removed once sent.