Thinking about what to do is an action in itself. If you pause to think whether to brake or steer left to avoid a crash, you’re not doing either. If a SWAT officer pauses to think during the part of a raid when the most important decisions happen, people get shot.
Most optimal algorithms do not involve questioning their own validity. There are times when you design and optimize, and there are times when you execute. Downtime is only useful when you’re not up.
I actually expected Harry to cast the Killing Curse as a last ditch desperation/rage effort. He knew what it does, has seen the wand movements and pronounciation (in the Dementor dream), knew and had the required state of mind. That should be enough to cast it, as per Ch26 (“He is in his sixth year at Hogwarts and he cast a high-level Dark curse without knowing what it did.”).