Tom's Hardware has posted a rather thorough test of the top five browsers.
Opera is declared the speed winner, which is great, but there are also some questions popping up regarding a couple of the tests.
First, the slow loading of local pages is a cache bug which only affects local pages directly from disk (Opera always re-decodes images when loaded from disk). I'm sure it's something we can fix fairly easily, but in the meantime it's possible to try loading pages from a local web server instead as a workaround.
Finally, the memory tests may not tell the full story. Opera will run fine on systems that are low on memory, but will also use more memory when available in order to increase performance. Since their test system has 4GB RAM, one would expect applications to take advantage of that. Opera runs on a wide range of devices, from mobile phones to high-end PCs, so it has been designed to adapt its memory usage to the environment.
Perhaps their next test could take a look at how the different browsers handle a larger selection of environments with different amounts of memory?