TL;DR – the phrase “90% faster” is interpreted by people in two different ways:

- 90% less time
- The speed (ops/sec or km/h) is 90% greater

I strongly prefer #2 but due to the ambiguity this phrase should never be used.

Describing performance improvements exists at the intersection of mathematics and linguistics. It is quite common to use incorrect math to describe performance improvements, and it is possible to use incorrect, misleading, or just sub-optimal rhetoric to describe your math.

Consider this hypothetical press release:

AirTrain Inc. is proud to announce the new AirTrain-8000. This revolutionary new plane can fly from London to Seattle at an average speed of 7,700 km/h – a huge improvement over the 770 km/h of other jets. This drops the travel time from ten hours to just one, making the AirTrain-8000 90% faster than our competitors.

A press release like this would never be released. The new plane is ten times as fast as previous planes (7,700 km/h divided by 770 km/h), and no marketing team would allow this improvement to be summarized as “90% faster”, which to many people – probably most people – means “almost twice as fast.” And yet, when talking about computers – where ten-times speedups happen fairly often – this mistake is made quite frequently.

This abuse of percentages has made them meaningless for describing optimizations – we need to stop using them. The AirTrain-8000 is ten times as fast, full stop.

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