Big Sur on 21.5-inch Late-2015 iMac with 8GB RAM and 1TB HDD

A friend of mine has an iMac with those specs, already running Big Sur. Performance is terrible. Right away, it’s obvious that the main problem is RAM, which is not upgradable. 3.5 GB out of the 8 are in use with just the Finder running.

Our one question is: would he see an improvement (like a reduction in spinning beachballs) by running the system from an external SSD instead of the stock 1TB HDD?


From everything I’ve read, the answer is “yes”. Modern versions of macOS (going back to 10.13 (High Sierra), while others may say going as far back as 10.9 (Mavericks)) hit up system storage pretty hard and hard drive speeds can cripple system performance, especially on systems with less RAM. (My 2011 Mac mini ran 10.12 Sierra OK with hard drives primarily because it had 16 GB of RAM and could cache the most frequently used parts of the disk, but it would’ve been terrible with its original 4GB).

As for how much performance will improve, it will depend on the type of SSD.

If your SSD is SATA-based, it will have a theoretical top speed of 6 Gbit/s, which may be further limited by the connection to your Mac (USB 3.0 maxes out at 5Gbit/s, while USB 3.1 gen 2 can go up to 10Gbit/s).

If your SSD is NVMe-based, then it can go much faster. If connected via Thunderbolt (most expensive, but fastest interface), you will see the best performance.

I can’t really suggest specific brands/models, but they have been reviewed here recently, so a search should reveal some recommendations.

I would strongly suggest getting/building a unit that has a cooling fan in the enclosure. High performance SSDs can run hot, which can lead to thermal throttling or device failure if they overheat. A case with a fan will prevent that from happening.

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Any system using the APFS file system instead of the older HFS will be horribly slow on a spinning drive. APFS makes no allowance at all for the way HDs do things. This means High Sierra up. It may start out ok on a fresh install, but the more it’s used the worse it gets. RAM is quite likely not the most important issue.

A system running from SSD even over USB 3 should make a big improvement. If budget allows and you over spend a bit to get a good SSD and an enclosure that can do USB-C 3.1 Gen 2 (10Gbit/s) it won’t make a difference now, but will be even faster on a new computer once than happens.

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I have that iMac but with 16GB of RAM. I have installed Big Sur on an external Samsung T5 SSD and my experience is that the system runs much faster than Mojave on the internal Fusion Drive.

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How does the T5 hold up as your system drive?

Looking at its product page, I don’t think it has a fan. I would be concerned about it overheating when used as a system volume instead of for occasional use as a data volume.

On the other hand, according to Toms Hardware, it is effectively a SATA device inside that enclosure, so maybe it never runs fast enough for overheating to be a concern.

It gets hot, but I haven’t noticed any performance issues. I’m only using it as a test bed while I evaluate Big Sur so maybe not stressing it that much.