On my machine I ran the SunSpider javascript benchmarks versus Firefox, Chrome and my JavaFX 2.2 webkit and the performance difference was staggering. For brevity, I'll only include the synopsis. All tests are on a pretty powerful i7 processor with 12G of RAM. My javafx application had plenty (3G) and the tests were conducted on Windows 7.

Firefox 15.0.1:

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RESULTS (means and 95% confidence intervals)
Total: 191.5ms +/- 2.5%

Chrome 21.0.1180.89 m:

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RESULTS (means and 95% confidence intervals)
Total: 220.0ms +/- 13.3%

JavaFX 2.2 shipped with JDK7u6:

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RESULTS (means and 95% confidence intervals)

Total: 1106.1ms +/- 0.2%

Roughly 5 times slower than the other browsers.