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My mini is a 2011 i5 with Radeon graphics. Most of my work is on an external rail via thunderbolt, but the system has always been a little laggy to me. I work on Macbook pros and MacBook Airs (new, with flash drives) at work, and the mini has never been as quick and responsive as them. I finally upgraded the disk to a SSD hoping for some better performance, and I bumped the ram to 16GB– but it got much worse with Yosemite. Downright aweful. After some googling I ran a cross a suggestion: Enable “Reduced Transparency” in the Accessibility Pref pane, and it’s made a huge difference. If you have a mini and 10.10 do it.