percentile is most common but most important term in performance testing process. if we are evaluating application or system performance then this is one of best way to understand where you stand currently.
Let's understand what exactly is 90th percentile mean ?
suppose we have a applications like below
First we need to sort the figures in ascending order as per above screenshot and then identify the percentile value with formula given below.
90th percentile value = 4.8*(90/100) = 4 (percentile value is 4)
So, conclusion is 90% of the applications are under 4 ms response time and 10% is on higher value. troubleshooting of any system or application etc will be consider on average value. whatever the average value come through this analysis, will be consider for further consideration. you are in green zone until unless cross the demand figures.