Snow Leopard is purported to provide many small but much-needed tweaks to its predecessor, Leopard. One oft-touted tweak is a speed boost, but according to tests by Macworld the performance and speed of a few different computers improved only slightly with many native tasks, and some took even longer.
Macworld installed Leopard and Snow Leopard on even-sized partitions on the drives of three different configurations: a 20-inch 2.66GHz iMac Core 2 Duo with 2GB of RAM, a 3GHz Xeon 5300 eight-core Mac Pro with 4GB of RAM from April 2007, and a 15-inch 2.8GHz MacBook Pro Core 2 Duo with 4GB of RAM. They charted simple tasks like start up, shut down, PDF scrolling, and more complicated ones like iMovie import/export and Photoshop CS4 filters. You can see the final results here.
The chart is a bit confusing about the actual speed improvement, and it is important to note that a mark of 100% on the chart indicates that the task performed was the same on both operating systems; likewise, a mark of 103% means it the task was 3% faster with Snow Leopard, and so on.