To compare the performance, here is the result running SunSpider benchmark with our demo browser in Qt 4.4 and Qt 4.5, the latter with and without JIT. The test machine is a typical four-core desktop running OpenSUSE 10.3. The test duration is converted to the number of runs per minute. Longer is better.
Let us try another one: V8 benchmark suite. The raw scores are shown here, again longer is better.