Safari is multi-threaded as may even be using GCD on Snow Leopard. You don't need multiple processes to leverage multiple cores. Note inter-process communication overhead, etc. can easily make multi-process implementations slower then equivalent multi-threaded ones.Yes but are you running on an 8 core system? This is where Chrome has the advantage. It runs each tab in it's own process which then can run on it's own CPU core. If you have a bunch of web apps all running at once and had a multi-core computer Chrome would win by a huge margin. THIS is why Google developed Chrome. They know that soon everyone will have 4-core CPUs and they hope everyone will want to rn their web apps
Google went the multi-process path to improve on security (isolate pages in tabs from each other) and resiliency (render crash in one tab doesn't bring down all tabs). Safari also uses additional processes for plugins like Flash (at least on Snow Leopard) to improve resiliency of the browser (plugin crash doesn't kill the browser).