In controlled JavaScript Web site renders, the iPhone 3GS is nearly three times as fast as the iPhone 3G and Palm Pre, and 5.5 times faster than the T-Mobile G1, according to a new study, which also reveals that the iPhone 3.0 software alone has a dramatic impact on the speed in which an iPhone 3G renders websites.

In releasing its new speed-centric update to the iPhone, Apple has boasted that the 3GS is twice as fast as its predecessor, but the company has remained mum on the handset’s actual specifications. Thanks to T-Mobile of the Netherlands, though, we know with certainty that the 3GS sports a 600MHz processor and 256MB of RAM – this compared to the 400MHz processor and 128MB of RAM on the iPhone 3G.

Based on the report released Wednesday from Medialets, a smartphone-based advertising and analytics platform, the iPhone 3GS spec bump far exceeds Apple’s “twice as fast” sales pitch in real-world tests. Medialets’s test aimed for a fair way to compare each of the major smartphone platforms. Because they all run very different operating systems, to compare their ability to run applications would be inaccurate.

However, all three platforms run browsers based on the open source WebKit standard. A MacBook running Safari was used in the test for a baseline comparison.

The chart illustrates a quite dramatic performance difference between the old iPhone and the 3GS.