These benchamrks are informal and only intended to give us a general sense of the benefit Dart Sass could provide relative to other implementations.

This was tested against:

libsass 014a61b and sassc 014a61b.
Dart Sass c63c440 on Dart 1.19.0-dev.7.0.
Ruby Sass e79f5cf on Ruby 2.2.4p230.

Measurements

I ran five instances of each configuration and recorded the fastest time.

Small Plain CSS

Running on a file containing 4 instances of .foo {a: b}:

sassc: 0.003s
Dart Sass from source: 2.219s
Dart Sass from a snapshot: 0.154s
Ruby Sass with --no-cache: 0.135s
Ruby Sass with a hot cache: 0.136s

Large Plain CSS

Running on a file containing 2^17 instances of .foo {a: b}:

sassc: 1.192s
Dart Sass from source: 2.705s
Dart Sass from a snapshot: 2.649s
Ruby Sass with --no-cache: 17.429s
Ruby Sass with a hot cache: 14.171s

Based on these numbers, Dart Sass is approximately:

2.2x slower than libsass
6.6x faster than Ruby Sass when it has to parse as well
5.4x faster than Ruby Sass with a hot cache

Preceding Sparse `@extend`

Running on a file containing .x {@extend .y}, 2^17 instances of .foo {a: b}, and then .y {a: b}:

sassc: 2.153s
Dart Sass from a snapshot: 2.766s
Ruby Sass with a hot cache: 21.843s

Based on these numbers, Dart Sass is approximately:

1.3x slower than libsass
7.9x faster than Ruby Sass

Following Sparse `@extend`

Running on a file containing .y {a: b}, 2^17 instances of .foo {a: b}, and then .x {@extend .y}:

sassc: 2.190s
Dart Sass from a snapshot: 2.722s
Ruby Sass with a hot cache: 21.970s

Based on these numbers, Dart Sass is approximately:

1.2x slower than libsass
8.1x faster than Ruby Sass

Preceding Dense `@extend`

Running on a file containing .bar {@extend .foo} followed by 2^17 instances of .foo {a: b}:

sassc: 6.542s
Dart Sass from a snapshot: 3.816s
Ruby Sass with a hot cache: 39.099s

Based on these numbers, Dart Sass is approximately:

1.7x faster than libsass
10.3x faster than Ruby Sass

Following Dense `@extend`

Running on a file containing 2^17 instances of .foo {a: b} followed by .bar {@extend .foo}:

sassc: 6.571s
Dart Sass from a snapshot: 3.586s
Ruby Sass with a hot cache: 40.705s

Based on these numbers, Dart Sass is approximately:

1.8x faster than libsass
11.4x faster than Ruby Sass

Conclusions

Based on this (admittedly imperfect and non-representative) data, Dart Sass is well within the desired performance bounds for large codebases. Because it eagerly tracks data for @extends, its worst case is when no @extends are present and that tracking proves unnecessary. However, even there it's only 2.2x slower than libsass, and well within a reasonable amount of time to process over 130,000 selectors.

Because of the novel structuring of @extend, we see its relative performance increase along with the amount of extension. With only one @extend it's almost on par with libsass; with hundreds of thousands, it's actually faster.

It's worth noting that Dart Sass implements @extend semantics according to [issue 1599][], while other implementations do not. This certainly simplifies the implementation and may explain some of the speed gains. However, even if other implementations could be faster, it's still the case that Dart Sass is fast enough.