2. Bluster while I prep. A lot of quick thinkers are not actually quick thinkers. They are quick responders, using far more words to say just as much, with the filler works frontloaded to give them time. For example:
"Now, correct me if I am wrong, and I may be, and this is something to consider, what if we X?" buys you about 5 seconds of time to think. You can say those words in front of pretty much every argument.
3. Stop caring. Few quick decisions are actually needed. If my Product Owner is going to make me defend my approach, I just concede the argument and allow the other guy's approach, whether or not it has gaping security holes or will fail in prod. Haven't made a case for anything at work in two years and just make sure everyone whos who did make the screw up.