"My experience was similar to yours. I interviewed at YouTube, and I was flown over from New Zealand for the interview. When I returned the next day, I got my generic rejection letter with Dear {Full Name}. While I got the general solutions to the 5 questions throughout the day correct; I struggled to code them on a whiteboard with someone watching me; I can never think well in those situations when solving problems, but I find that the optimal solution clicks to me when I'm in the shower or wake up in the middle of the night."

I agree wholeheartedly, see my above comment about optimizing a solution being more of an art, and can take a varying amount of time.