Does that slump correlate with meal times, or is it completely disconnected from your eating schedule? About 2-3 hours after eating may be when the insulin from the meal drops and you can really hit the fat burning. If your fat energy mechanism is not 100% yet there can be an energy drop.

Are you a lark or an owl? (most alert first thing in the morning is a lark, later in the day for an owl) I'm a lark and my energy slump is around 3 independent of meals.

What is your overnight sleep like? mildly not enough or a lot not enough?

Might you have sleep apnea and not know it? I did.