To measure this I built and open sourced the “CSS Selector Benchmark” project which I have been working on for some time now.
https://github.com/GoogleChromeLabs/css-selector-benchmark
The test suite is powered by Chromium’s PerfTestRunner and Puppeteer.
The created benchmarks for @property specifically measure how fast Blink can handle a Style Invalidation and the subsequent Recalculate Style task.
This was tested with both registered and unregistered custom properties, as well as regular properties.