While the how well part can, in general, be described in natural languages,
in our limited scope, the focus is on something that is quantifiable as a real
number. Real numbers include integers and 0/1 binaries as special cases.
Natural language descriptions are still very important. For example, a news
article that heavily criticizes Flutter’s performance by just using words
without any numbers (a quantifiable value) could still be meaningful, and it
could have great impacts. The limited scope is chosen only because of our
limited resources.

The required quantity to describe performance is often referred to as a
metric.

To navigate through countless performance issues and metrics, you can categorize
based on performers.

For example, most of the content on this website is about the Flutter app
performance, where the performer is a Flutter app. Infra performance is also
important to Flutter, where the performers are build bots and CI task runners:
they heavily affect how fast Flutter can incorporate code changes, to improve
the app’s performance.