processing speed is hardware driven. For example, your OG droid is clocked at a certain speed (off the top of my head i don't know, think it was somewhere around 750mHz? i'm sure i'm wrong) and you have the newer phones which are clocked to say 1Ghz. This affects the speed in which apps open, smoothness of the phone, and overall performance. There are other things that come into play for processing speed, but I'm trying to keep it as simple as possible.