which OS to install on a late 2013 21" iMac with 8GB RAM and HDD?

When "slow" comes up here, it can be hardware or software related. If it is the latter, fixing the issue can be free so best practice is to identify and eliminate possible software problems first.

Anti-virus apps, so-called "cleaning" utilities, and torrents can make a Mac slow. Consider asking your friend get EtreCheck from the Mac App Store ( https://apps.apple.com/us/app/etrecheck/id1423715984?mt=12 ) to review her system config for known issues. It is safe and will reveal no personal or secure information. It will also report some hardware issues like poor hard drive performance.

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My impression is that High Sierra, Mojave, and Catalina won't run well on a late 2013 iMac without a SDD.

We have an even older 21.5" 2011 iMac with 8GB RAM and High Sierra still in daily service. Once we get past the slow launch times of the 3GBps mechanical hard drive, the computer runs fine.

The external SSD option, not available for our 2011 model, is a great option for your friend to address the slow load time. Currently she is "enjoying" hard drive write/read speeds of probably no more than 70 MBps. People here using the external SSD option on similar iMacs are reporting write/read speeds in the range of 400 MBps.

beethoven_girl wrote:

My impression is that High Sierra, Mojave, and Catalina won't run well on a late 2013 iMac without a SDD. Which Mac OS runs best on a late 2013 iMac with 8 GB RAM and no SDD?

A 21.5" iMac has a very slow HDD. Basically any version of macOS will be quite a bit slower than it would be on an SSD.

I have a late 2016 13" Touch Bar MacBook Pro 3.3GHz i7 / 16GB RAM / SDD, and I'm also wondering whether upgrading from Sierra will benefit me. I'm generally disinclined to upgrade unless absolutely necessary. I tend to have a zillion tabs open in Chrome, Safari, and Firefox at any given time, and I'm frustrated with how much this slows my MacBook down.]

Basically if you have "a zillion tabs" open at a time it's going to run slow. FWIW, Chrome uses more resources than Safari or Firefox so it will slow things down more.

The main advantage in newer versions would be better security. Catalina does not support 32bit apps.

Microsoft's Web site on OS requirements:

macOS: Office for Mac is supported on the three most recent versions of macOS. As new major versions of macOS are made generally available, Microsoft will drop support for the oldest version and support the newest and previous two versions of macOS. Product functionality and feature availability may vary on older systems. For the best experience, use the latest version of any operating system specified above.

This would indicate Catalina, Mojave, High Sierra

With 8GB of RAM and with only a rotational hard drive I'd not recommend Catalina and Mojave would be noticeably slow too. That said, an SSD in a good USB 3 external enclosure will provide a huge speed bump making this computer capable of running Catalina and Office.

beethoven_girl wrote:

A friend of mine, who is a light computer user, just "inherited" a 21.5" iMac 2.7GHz i5 / 8GB RAM / 5400 rpm HDD. It's currently running Yosemite, but she wants to install Microsoft Office 2019. MS says Office for Mac is supported on only the three most recent versions of macOS. Will Office 2019 install on an earlier OS? My impression is that High Sierra, Mojave, and Catalina won't run well on a late 2013 iMac without a SDD. Which Mac OS runs best on a late 2013 iMac with 8 GB RAM and no SDD?

Your friend really needs an iMac with better spec's (16GB and SSD) to upgrade past High Sierra and run MS Office at the same time. High Sierra or Mojave would run OK on your friends iMac, but if they have never purchased High Sierra or Mojave from the App Store, then the App Store may only allow that iMac to upgrade to latest macOS Catalina.

[I have a late 2016 13" Touch Bar MacBook Pro 3.3GHz i7 / 16GB RAM / SDD, and I'm also wondering whether upgrading from Sierra will benefit me. I'm generally disinclined to upgrade unless absolutely necessary. I tend to have a zillion tabs open in Chrome, Safari, and Firefox at any given time, and I'm frustrated with how much this slows my MacBook down.]

Your 2016 MBP would do fine running Mojave or Catalina. You just need to decided on one browser and stick to using only that browser.

As I see it, it comes down to (1) how 'fast" do you need your computer to run, and (2) is it used with apps such as Adobe provides which require a lot of computing power? In other words, fast can be subjective. For the casual user your computer should be ok running Catalina. I had the same computer with all the latest Microsoft apps and it was not real fast by any means, but it was acceptable. I was able to compare it to my wife's new 27" iMac and see the difference. My computer ran just fine, but I did eventually decide to upgrade because I realized that due to its age, its HD wasn't going to last forever. So, I traded it in with Apple and now someone else has it.

A few months ago, I have installed Mojave on a 21.5" late 2013 iMac. I works well.