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I recently bought a 2nd-hand iMac which has the following specifications:

  • iMac (27-inch, Mid 2011)
  • Processor: 3.4 GHz Intel Core i7
  • Memory: 16 GB 1067 MHz DDR3
  • Graphics: AMD Radeon HD 6970M 2048 MB
  • Storage: 1TB SATA HD

It had Sierra installed, and I used it for a couple of weeks.

I then upgraded to High Sierra and noticed that it is very slow.

It seems to lag everywhere, even when inputting text into this text field in Safari. The lag is not extreme, but it is there.

I assume this 2011 iMac has insufficient processing power to smoothly run High Sierra which was released in 2017, so I would like to downgrade to an earlier OS.

I considered downgrading to Sierra, but I wonder whether it would be even better to downgrade to an earlier OS such as El Capitan or earlier.

From the MacOS release dates on the Wikipedia page, I assume this iMac was released with Lion.

I use XCode 9 and not much else, and I assume this iMac has a 64-bit processor, and I don't know which was the first OS to support 64-bit.

I'd like to downgrade to the earliest version that I can, to ensure the smoothest experience.

So which OS should I downgrade to?

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    Your processor and RAM is more than good enough to run High Sierra. It is way more powerful than my tiny 12-inch MacBook that I currently use, and even it can run High Sierra just fine. Your problem is the spinning rust (aka hard drive) it's installed on. Replace that with a solid state drive and you should be good to go! – André Borie Jul 26 '18 at 8:40
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    @AndréBorie Why would the speed of the hard drive affect entering text in Safari? – James P Jul 26 '18 at 9:01
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Considering your usage, the answer is fairly straightforward:

  • Xcode 9.0 to 9.2 require as a minimum macOS Sierra 10.12.6
  • Xcode 9.3 to 9.4.1 require macOS High Sierra 10.13.2

So you're basically limited in your options to these two versions of macOS.

You didn't specify the exact version of Xcode you have installed, but you may need to change that to go down to macOS Sierra. Also, you may need to consider your long-term usage, as Xcode 10 (still in beta) requires macOS High Sierra 10.13.6 and it won't be long before a version will require macOS Mojave.

If you decide you want to keep the iMac, then your best bet would be to upgrade to an SSD (either replacing the optical drive, or the internal HD). I suppose another option would be to get an external SSD which supports a Thunderbolt 1 connection and then install macOS on that to use as your main boot disk. While a little clumsy, it's still going to result in a better experience all around.

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