jsPerf.com lets you write two equivalent ways to accomplish something in JavaScript and it then measures how fast each method is in every browser you run the test in. Other people’s performance tests can be browsed here Browse test cases.

Taking a look at a specific test could show data where you might want to pick an approach that is 25% slower in Chrome …

For example operator-vs-localecompage shows that FireFox starts out seven times slower than Chrome and the slower method in Chrome runs twice as fast in FireFox as the faster method in Chrome.

Other people who view your test can contribute new results when it is run in their browser also.