With only defaults Opera is faster. Take away Fasterfox and some of the other extensions in Firefox and it is not as fast.
Testing on my rig right now Safari 3.1 is the fastest. Opera 9.50 is 2nd fastest and Firefox 3 is 3rd. IE8 is the slowest by far.
Each system responds differently. There is no way to conclude that 1 browser is faster than another since each system will respond to each browser differently.