Anandtech went through the trouble of running various speed-based benchmarks on the iPhone 3GS and Pre, clocking the time it took to launch various apps and load web pages. Though they share similar hardware, the iPhone 3GS was deemed "faster."
In general, the iPhone 3GS was about 20% faster than the Pre in any given test, loading all but a couple of web pages quicker. And though it's less accurate of a benchmark, the iPhone launched its core apps 3-4x faster than the Pre on average. While the two phones share the same platform, the iPhone 3GS runs on the Samsung SoC chipset and the Pre runs on the TI OMAP3 chipset.
The iPhone 3G was also in the benchmark mix. When it came to loading web pages, the 3GS was about 1.2x faster than the 3G (3x for some pages), and when it came to launching apps, it was anywhere from 1.4x-7.2x faster.
But to be fair to the Pre, in an earlier article, Anandtech offered up a couple of explanations why the 3GS might perform better in raw speed tests: first, Mobile OS X has been around longer, therefore having better and more thorough software optimization. Secondly, Anandtech speculated that the Pre might be prioritizing code around multitasking capability instead of raw speed, which works against the Pre's favor in these clock tests. Anyways, interpret these numbers how you will. [Anandtech via MacRumors]