I run computers that have to be the least bare minimum to run any of these browsers for example I use a computers that has 128 Mb ram 350 MHz celeron processor with windows xp. In this setup Internet explorer is miles ahead of others in load time and speed. For example it takes like a second to or two for ie, where as firefox and opera take like 30 to 60 seconds. If only internet explorer was a bit more secure.
Here is an interesting video that show that Internet explorer is faster than firefox.