There's optimisations for repeated execution of the same commands at probably every level, from the interpreter down to the hardware on the server side, and the

There's optimisations for repeated execution of the same commands at probably every level, from the interpreter down to the hardware on the server side, and the browser on the client side will cache and preload things smartly in the background. Usually pressing reload (F5) only downloads modified content though, to ignore the cache you have to do a force reload (ctrl+F5 I think!)