A giant problem is that people want to see reported what their 95% latency is. This is a complicated number that does not aggregate easily. Because as the article says, you need the whole dataset to calculate it.<p>By contrast we can ask what percentage of records is over 400 ms. This is a simple number. And if we simply keep track of how many requests there were and how many are slow, this is a simple thing to aggregate and a simple thing to calculate.