I can speak from my experience. I was for a long time drinking 2-3 cups of coffee in the morning. During the morning, generally things were good and I could crank out code. However, during early-mid afternoon I'd often get overwhelmed, tired, and unfocused, especially when multitasking, and often would lapse into a browse-reddit/agh-i'm-not-being-productive/browse-more-reddit cycle. Cutting down to one 1 cup in the morning has made a real difference - I no longer get the early afternoon slump, and can stay focused for longer in the afternoon without being overwhelmed by a long task list.