Are you a Nexus Device user having syncing issues? If you are, this is one of the issue that is cropped up. Multiple people have been suspecting a Google sync error with receiving emails. This error, in particular, has been hitting the Nexus uses running Marshmallow. People are complaining that their device is not alerting them of incoming emails right away and if the notification comes, its packed with a delay in the system. Google is aware of this issue but if you are looking for temporary fix, clearing the app data and cache may fix the problem.
However, Google has replied to this thread and have said that “We have passed this defect on to the development team and will update this issue with more information as it becomes available. Thanks.”