# Latency numbers, and where the time actually goes

- Source: https://intervueclub.com/course/latency-numbers
- Author: IntervueClub
- Part of: HLD 101 (https://intervueclub.com/course), Thinking in systems
- Last updated: 2026-09-19

The latency numbers worth memorising, why tail latency matters more than the average, how to build a latency budget across network, cache and disk hops, and why fan-out makes p99 the number that decides your design.

## Orders of magnitude, not exact figures

Nobody will ask you to recite a table. What they will do is watch whether you notice that a design puts a 150ms cross-region call inside a 100ms SLO. That only requires knowing the rough scale of things, and the scale is remarkably easy to remember because it moves in steps of roughly a thousand.

| Operation | Time | In human terms |
|---|---|---|
| L1 cache reference | 1 ns | 1 second |
| Main memory reference | 100 ns | 2 minutes |
| Compress 1KB | 2 µs | 30 minutes |
| SSD random read | 100 µs | 1 day |
| Read 1MB sequentially from memory | 250 µs | 3 days |
| Round trip within a datacentre | 500 µs | 6 days |
| Read 1MB sequentially from SSD | 1 ms | 12 days |
| Disk seek (spinning) | 10 ms | 4 months |
| Read 1MB from network | 10 ms | 4 months |
| Round trip, California to Netherlands | 150 ms | 5 years |

The right-hand column is the one to internalise. If a memory reference took a second, going to disk would take four months and crossing an ocean would take five years. That intuition is what makes a bad design feel wrong before you have done any arithmetic.

Three consequences fall straight out:

**Memory is effectively free; the network is not.** This is why caching works at all, and why the entire caching concept later in this track is really about avoiding hops rather than avoiding computation.

**Sequential beats random by a lot.** Reading 1MB sequentially from SSD costs about the same as ten random reads. Data layout — clustering rows you read together — is a performance decision, not a tidiness one.

**Geography is a hard floor.** Light in fibre does about 200,000 km/s. London to New York and back is roughly 11,000 km, so about 55ms of pure physics before any processing. No engineering removes it. You can only avoid making the trip.

*The rest of this concept is free to read after signing in: https://intervueclub.com/course/latency-numbers*