Once a pixel is fired it takes our system about less than 100 milliseconds to process and call Zendesk's API. There are two reasons why your read receipts might be delayed.
First, Zendesk receives millions of API calls per day and has to distribute the processing workload and there may be limits to the number of API calls they can process depending on their software architecture which we are not privy to.
Second, from time to time, Zendesk has service outages for various pods which may cause our API calls to them to fail. In this situation, we store all of the pixels until the service is restored and then we post those pixels.