6.0 Marshmallow on HTC M8 Gmail sync is very slow, how can I speed it up?

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AC Question

6.0 Marshmallow on HTC M8 Gmail sync is very slow

Gmail app is slow to sync new emails. It can take an hour or more for the sync to happen and give me a notification. Not sure if Doze has a role but seems like it. Never had an issue on KitKat. This only started after going to Marshmallow. I only briefly ran Lollipop (1 hour!).

Any setting I can use to get gmail to sync frequently? I have excluded gmail from Doze.