You may have a problem in your math. A mean of 4.3us with a stdev of 10.9us should mean that your 99% should be over 26us, unless it's super smooth with a few huge outliers.
Also, are you sure you didn't mean ms and not us? Seems more likely for an HTTP service.
Also, are you sure you didn't mean ms and not us? Seems more likely for an HTTP service.