Why p95 and p99 Latency Matter for Reliable SystemsWhy p95 and p99 Latency Matter for Reliable Systems
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A system can be fast most of the time and still create a bad experience when it matters most.
Imagine two services.
One usually responds in 5 milliseconds, but occasionally takes 500.
The other usually responds in 7 milliseconds and rarely goes above 12.
The first looks faster if you only look at the average.
But under real traffic, those occasional delays can become the difference between a smooth experience and customers waiting, requests timing out, or downstream systems backing up.
This changed how I think about performance.
I no longer ask only:
“How fast is the system?”
I also ask:
How consistent is it under load?
What happens to the slowest 1% of requests?
Does performance degrade when traffic increases?
Can one slow dependency create delays across the system?
This is why p95 and p99 latency matter.
Good performance engineering is not just about making the average request faster. It is about making the system predictable when conditions are less than ideal.
Enoch's avatar
Average performance can hide a lot.
If 99% of requests are fast but the remaining 1% are extremely slow, that can still represent thousands of poor experiences at scale.
That is why I increasingly care about the shape of performance, not just the headline number.
Predictability is...
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