JSS has some performance advantages.

• Incremental compilation and rendering (as soon as needed).
• Rendered styles are cached. Compilation and DOM Rules creation happens only once.
• Only styles which are currently in use on your screen are also in the DOM (react-jss).
• Simple class selectors ensure high selectors performance at scale.

These benchmarks are used to ensure performance after code changes.

However, one test compiles bootstrap library from JSS to a CSS string. On my machine in Chrome ~130KB JSS compiles at 180 ops/sec.

yarn
yarn bench

css-in-js-perf-tests

JSPerf bench.

JSPerf bench.

Inline styles are also slower because of a simple fact that same styled items (e.g. in a list) don't share the same rule.

JSPerf bench 1

JSPerf bench 2