My entire system, as a whole decreases over time after a freshly installed OS. A year later or so things just aren't quite as snappy. I would think this to be inevitable and applied to everyone. It's not like it gets slow by any means, but not quite as snappy as the day I did the format. I reformat my box every 2 years for this reason. And after I'm done throwing everything back on it, I can definitely notice it's snappier.