Memory Upgrade for my iMac (Retina 5K, 27-inch, Mid 2015)

Why do you want to upgrade the memory? If it's to speed up your iMac that won't do it. The boot speed, speed of opening and saving files is determined by the speed of the boot drive. If you have an SSD drive then that's the fastest you can get. If it's a Fusion drive that will be a lot slower than an SSD. And finally an HDD is will glacially slow compared to an SSD.

If you want to increase memory so you can open and work with larger images or documents then memory will help. I've had 16 GB with an Intel 27" i9 CPU and a Silicon Mini and it seems fine for what I do. If you're doing some heavy image or video editing 24 GB should do the job. The only place I would get memory is from OWC (MacSales.com). Macs are extremely fussy about memory quality and OWC is the only vendor which most of us here will use. They test modules with the Mac they are designed for to verify compatibility.

The speed is 1600MHz.

A good supplier for high quality and reliable RAM for Macs is OWC or MacSales.com, also crucial.com, but Crucial no longer stocks ram for a 2015 iMac unfortunately.

MacSales:

https://eshop.macsales.com/item/OWC/1600DDR3S32S/

at Amazon.com: OWC 16GB (2 x 8GB) PC14900 DDR3 1866MHz SO-DIMMs Memory RAM Upgrade Compatible with 2015 (Late) iMac 27 w/Retina 5K Models - Amazon.com

There may be other options out there, but YMMV with those.

For example:

Timetec 32GB KIT(4x8GB) Compatible for Apple Late 2015 iMac (27-inch w/Retina 5K Display) DDR3L 1867MHz / 1866MHz PC3L-14900 - Amazon.com

Try the following:

1 - download and run the free version of DriveDX and post the report with your reply or a screenshot of the DriveDX's window.

2 - download and run the free version of Etrecheck according to this user trip: How to use Etrecheck.

DJ_MARKY_MARK wrote:

My iMac is in mint condition and still has 700GB+ storage on it, but for some reason it has started running very slow

If you have not recently started running much heavier workloads, that might tend to point to a cause other a lack of RAM. I used a Late 2009 iMac with 8 GB of RAM for many years, and for most applications, the amount of RAM did not seem to be an issue.

Except when I ran Parallels Desktop and started up Windows inside of it. Then Windows very painfully, very slowly, staggered to its feet. Later, when I added an external SSD, starting up Windows went from "root canal type slow", to "merely dog slow." That was a case where the Mac clearly could have used more RAM. If you are a moderate to heavy user of programs like Adobe Photoshop, that might be another case where >8 GB of RAM would help.

But I'm thinking that perhaps your hard drive (or the hard drive component of a Fusion Drive) is beginning to fail. Or that you've installed some sort of "antivirus", "security", "cleaner",etc. software that might have contributed a lot to the sudden slowdown of the system. In this case, adding more RAM is unlikely to help much unless you also fix the underlying problem.

, i've tried doing a software update but its not even finding the update from the apple site when I click get and the iMac it keeps telling me in preferences I need to do software update

You can upgrade that iMac as far as macOS 11 (Big Sur), and no further. I would not suggest doing so until you have checked out the health of the drive a little more – as all of the drive access involved in installing a new version of the OS could push a failing drive (IF it is failing) over the edge.

How to download and install macOS - Apple Support