I've got a late 2011 MBP. It had OSX Lion on it originally and I upgraded via the Mac App Store to Mountain Lion, Mavericks and then Yosemite as and when they were released each year. I'm aware that with Windows, it slows down over time and it's good practice to format Windows based systems and reinstall the O/S ever year or so if possible. This makes quite a difference in terms of performance. Do Macs also benefit from improved performance after a format/reinstall? My Mac doesn't run too slow - I'd say it's a bit sluggish after first booting but that might be because it has 4GB RAM. I remember when I first got it in 2011 it was very fast and programs opened almost immediately. Since I've had the Mac it hasn't really had much software installed - just programs like Numbers, Pages and one or two others. Everything else is stock installed. I've got a 6GB iPhoto database which is pretty slow to open.