Messages taking a long time to appear When I receive a message there will immediately be a notification, however when opening the Messages app, the message will usually take about 5 seconds before it appears.

If there a known issue with the OS or a quick fix for this issue?
iMessage and iCloud have lots of issues with delivery, sync, and reliability. There's no fix, but you can always write feedback to Apple or file a bug report (free developer account required).