Same issue as this 2015 post I think Extreme Workflow Email Delays .
I'm working on a set of workflows in a particular site. The workflows run to completion within a minute of being started, but email notifications from those workflows are taking hours/days to arrive. Other workflows in the site collection are running normally (emails arrive within minutes). Not sure if I can isolate this to a particular site yet or not. These are all list workflows working off the same list.
Looking for suggestions on how to proceed. Thanks.
