Some Mac users are experiencing significant delays in receiving iMessages and SMS texts
because of a bug affecting macOS High Sierra, as first spotted by AppleInsider. The issue reportedly also stops iCloud-connected devices like iPhones and Apple Watches from notifying users of received messages whenever the Mac is active.
One user wrote on an Apple forum, "I have to manually click the home button to see if I have received any messages. Only when i shut down the Macbook does the phone start responding with sounds + vibrations." For users affected, the only workaround seems to be completely disabling iMessage from their Mac, or downgrading to macOS Sierra. Other options, like disabling then reenabling iMessage, or sending a message on the Mac seem to only fix the problem temporarily. https://www.theverge.com/2017/10/18/16492180/imessage-delay-bug-macos-highsierra
