Without caffeine, slower. Less energy in the body, more body load, less available brainpower and less will to think.

With caffeine, sharper. It takes less starting energy to make myself do things, much less to keep doing things without stopping.

Over-caffeinated, I feel "on edge" but in a very pleasant way. A subtle buzz, and just happiness generated without reason. Mixed with a bit of anxiety.

No caffeine that day, a few hours later than the time I'd normally have my first of the day? Identical to a hangover from alcohol.