am i just slow or is my brain not working

No, you're not slow. It should have been run at 250 ml/hr to infuse the whole bag over 60 minutes. Likely, the nurse was worried whether the IV site could handle the rate.

You'll feel slow for quite some time. I've always been that way too whenever I'm put on the spot. The answer could be screaming in my head, but at the same time I'm terrified I'm wrong & clam up. It will get easier!

I agree with nu rn. Most likely, the nurse was concerned about compromising the vein with a higher rate (250 ml/hr is pretty fast for a peripheral, especially if it's somewhere fragile like the hand). I personally never run antibiotics that fast, unless it's a central line or PICC line, for exactly that reason. Some antibiotics are naturally irritating or hard on veins to begin with.

This is a great example of nursing judgement, and is something you'll develop with more and more experience. Don't worry about the math, as doing rate calculations is usually more common with drips (Cardizem, etc.). These things get easier with time and experience.

Also, it sort of sounds like your preceptor nurse is orienting multiple orientees? If that's the case, I'd talk with your manager. Orientation is a critical time for a new nurse to learn with the direction of an experienced nurse. If this nurse is supervising 2 orientees, you're really only getting 50% of the attention you should be getting.