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Opera has the fastest JS engine now (Sunspider test)
Sunspider benchmark (Win XP, dual core machine):
Opera (v10.60): 279.6ms +/- 0.9%
Opera (v10.53): 299.8ms +/- 1.0%
Safari (v5): 314.2ms +/- 1.7%
Chrome (v5.0.375.55): 326.6ms +/- 7.9%
Firefox (v3.6.3): 716.4ms +/- 1.7%