Thanks to Obama Pacman, here's a neat (audio muted) video comparison of web browsing speed over 3G on an iPhone 4 (shown on the left) and an iPhone 3GS (shown on the right).
The test was done using only mobile 3G with Wi-Fi turned off, and it shows a variety of sites using Safari in a head to head comparison. In almost all the tests, the iPhone 4 was much faster in Web browsing.
When you look under the hood at the iPhone 4's 3G performance, the improved browsing speed is not much of a surprise; CNET benchmarked the 4 against the 3G (not the 3GS) and found dramatic improvement across the board. Even against the more recent 3GS, the iPhone 4 delivered stronger download performance in Andy Ihnatko and John Gruber's tests.
The processor is obviously kicked up a notch as well. MacRumors, running benchmark tests, found the iPhone 4 processor to be 31% faster than the 3GS using Geekbench 2.1. The iPhone 4 came in with a 2.514 score across routine tests like memory allocation and view drawing, easily besting the 3GS at 2.298. Both were tested under iOS 4.
Benchmark tests are not really analogous to real life usage, of course, but they do give a basic indication for comparison purposes. So although it probably takes more muscle to paint the screen using the iPhone 4's high-resolution Retina Display, the faster A4 chip combined with the improved network throughput (when you've got all your bars, that is) seems more than up to the task.
Apple iPhone 6
Apple iPhone 3GS
Apple iPhone 4