It's not at all clear what the root cause of the problem is, but we've noticed a big uptick in people complaining about performance issues on new 27-inch iMacs on Apple's support boards. The most common symptom seems to be incredibly slow Flash video playback, but people are also claiming that there's a bug forcing the hard drive to spin down, that there's a corrupted Snow Leopard build preinstalled, and even that the problem is bad permissions, which is basically how an OS X user gives up trying to figure things out. Apple hasn't said anything yet, but neither of the 27-inchers we have here are having problems, so we're keeping our fingers crossed while we wait for an official resolution. Video of the issue after the break.
[Thanks to everyone who sent this in]
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AppleiMac 27-inch with Retina 5K display (late 2014)