I am not so sure that the extra half inch slows me down that dramatically, but rather the style of the shoe. You have inspired me to experiment in the future (might have to wait until spring). I don't have very many non-stilettos that have a steepness of 4 1/2" or greater, but I think the reason why I'm walking more slowly in stilettos is because I have to be more careful how I walk and where exactly I step, so as not to damage my shoes. If I were to do the experiment at say, a shopping mall or some other place that is perfectly flat and has no cracks in the pavement, the difference might not be as much.
I have been rather motivated to set a new annual mileage record, and have walked about 11 miles on my three days off for the Christmas holiday. I now stand at over 308 miles for the year, or less than two miles of the previous record. I think I can manage to walk at least two more miles by the end of the year. For only the fourth time in my high heeling career, I did a five miler. 5.3 miles to be exact. Yes, my feet and legs were tired by the end, but I really suffered no ill effects. My left sandal started rubbing my instep slightly for the last mile, but it was a very tiny wound, and two days later I don't even notice it. Here is a picture of me right after the 5 mile walk. I am fortunate that the weather is unseasonably warm for December, about 45º F or 7º C.