Post the macOS 10.13 High Sierra update, many macOS users have encountered a bug which is leading to their incoming iMessages to be significantly delayed. For some, the situation is even worse as they are not getting any notifications for the incoming messages on their iPhone or iPad connected to the same iCloud account.
This bug likely persists due to Apple working behind the scene on the Messages in Cloud feature. As the name suggests, it automatically syncs all your iMessages across all your devices linked to your iCloud account. While the feature was a part of the initial iOS 11 beta, Apple removed it afterwards and mentioned that it would be available to users with a future iOS update. Since then, it has been presumed Messages in Cloud will make a return with the iOS 11.1 update which is expected to release around the time the iPhone X launches.
Until Apple gets around to solving the problem, affected users have come up with some temporary fix for the issue. This includes disabling and re-enabling the Messages app on your Mac or sending a message via your Mac. As mentioned though, this is only a temporary fix and the problem reappears after a few hours. The only fix that seems to be permanent in nature is to disable read receipts in the Messages app on your Mac.
The bug continues to exist in the latest beta of macOS High Sierra, though it is always possible that Apple gets around to fixing it by the time it releases the final public build of the OS.