reference — fundamentals
Performance vs scalability
in short — written for this site
A service is scalable if adding resources results in a proportional increase in performance.
symptom
Slow for one user with no load — performance. Fast alone, slow together — scalability.
test
Scalability holds when doubling the resources roughly doubles the work served, at unchanged latency.
from the primer — full sectionsynced
A service is scalable if it results in increased performance in a manner proportional to resources added. Generally, increasing performance means serving more units of work, but it can also be to handle larger units of work, such as when datasets grow.<a href=http://www.allthingsdistributed.com/2006/03/a_word_on_scalability.html>1
Another way to look at performance vs scalability:
- If you have a performance problem, your system is slow for a single user.
- If you have a scalability problem, your system is fast for a single user but slow under heavy load.
Source(s) and further reading
sourcedonnemartin/system-design-primer / README.mdanchor#performance-vs-scalabilitysyncedsynced from donnemartin/system-design-primer@master · 2026-08-16